To shortly answher your question :You can aproximate a proper bike to buy, but you can be sure only by actually riding it long enough.
Now, to elaborate my answer:
First of all, there is no such thing as a true "do it all bike". However, there might be a bike that perfectly suits all YOUR needs, as long as they are reasonably close. For an extreme example, ther will be no bike that allows you to jump rampage style 5 meter drops while being so lightweight that you can climb the Everest on it. Having that said, lets asume your needs are not to far from each other in the MTB activities spectrum.
Most people think suspenssion travel is the (only) key factor determinig the habilities of a bike, but actually other factors can be even more decisive.
Fork tuning: Any fork that has good adjustment capabilities, asuming your ideal setup falls inside the fork's range, will have also a configuration that you'll feel terrible. A decent contemporary fork should have at least two adjustments: Preload/Sag, rebound speed.
Preload and sag are intimately related, being sag the amount of travel taken by the fork by just aplying rider's weight to the bike. Every manufacturer will state an apropriate sag range for every fork model. Rider weigth affects sag, and you reduce it by increasing preload, or augment sag by decreasing preload. But there is a range (instead of a single solid number) for a reason: two diferrent people, riding the same bike will handle it with their own style, technique, etc... For example, when I descend a rocky XC trail, I do it standing on the bike, in a neutral posture and "helping" suspension with my elbows (and knees), therefore, I tend to use more preload (less sag) for a more precise handling, and a more responsive feel. My father, instead, will take the same trail seated, leaned back as fas as he can go, so he tunes his suspension towards the higher sag recomended value. Neither of us will feel comfortable/comfident riding each other's bike.
Rebound speed is how fast the suspension will return to its "normal" position after hitting a bump. Rebound speed is also intimately related to rider's personal technique, but also the ridden terrain, to the degree that some riders use a different setting for some trails than for others. In my case, as a DH rider, I usually set rebound according to the particular track, but as a XC rider, I just have set a rebound speed that is a fair compromise that will allow me to confidently ride most of the trails I use to ride.
Fork Design: Further than just travel, check what's the indended use and rider weight of your fork. There are, for example, 100mm XC forks, 100mm Dirt Jump forks, and I have seen 100mm forks for commuting/touring/hauling bikes. For each one different design choices have been made, and ones can be unsuitable for other uses. For example, a fork designed for a skinny rider may be imposible to adjust properly for a chubby one. Or a fork made for a hauling bike will be perceived "rock solid" if put in a lightweight XC bike for a small rider.
Rider related factors
Besides factors related to the fork, there are factors related to the rider. Note I have mentioned "style" and "personal technique" several times. These are characteristic that a rider develops with time and practice, but also, they evolve once established. Thus, a "proper" fork tuning is espected to change over time, specially for a beginner rider (even if beginer in one specific discipline and expert in another).
Changes in rider's body also affect performance, style, technique. These factors are normally changing, except maybe for stablished professional riders and a few other, but for most of us, "simple mortal riders", we are constantly gaining or loosing weight, aging, going through fatigue/recovering cycles, etc... All of them change "who we are" while on top of our bikes, so, the perfect bike one day, can be a terrible one a few months later, but may become ideal once again later. Consider in this category injury recovery time.
Other components of the bike
When you swap some component you may be changing bike handling characteristics. The most likely to interarct directly with rider's perception of the fork is the chosen tire. Some tires will contribute positively or negatively to handling. For example a hard rubber tire with low adhesion is not a good combo with a fast rebounding fork. But a soft tire, with good traction and specially a fat one that can be used with less presure may work wonders with the same fork setting. Rim, spokes and wheel lacing are also factors to include in the ecuation, though to a lesser extent.
Practical considerations
You mention "... start hitting the trails...", that leads me to think you are kind of novice. (Please excuse me if not the case). That means you will be growing in experience, technique, possibly getting more fit and also trying harder challenges, longer rides, faster racing, etc. With all of these you may find yourself wanting to change something on your bike, or even the hole bike, but you can not know for sure until you ride it long enough.
You get your new bike, make sure to "properly" fit and tune it, but remember: most setings and specific measurements for fitting and set up are given in ranges, you get to choose the specific value for each. Even so, many fitting rules are actually starting points, not unbreakable laws, so you can deviate slightly or not no slightly from them to suit your specific needs. Also, a single setting may not be enough for all the things you ride, sometimes you'll have to cope with the compromise, sometimes you'll have to plan ahead and tune your bike for what you'll be doing.
The same goes for all the bike, including the fork, so, don't get desperate if you find yourself making constant changes during the first months. As long as you ride enough, you'll find the sweet spot for every adjustment that can be made. So, if after riding enough, after learning about the practical limits of your bike and your body, you find yourself hitting the extreme of an adjustment dial, the maximum or minimum extension of something, then, start considering to change that component.
It is advisable to change one adjustment at a time, so you cope with less variables. Do the adjustment, test it by riding, do not just take a few loops in your backyard or in the street in front of home, cope with the differencies for a while and then evaluate if it's really better or worse. When you make an adjustment, I recomend: change it until you notice it is too much, then go back a little.
P.S. Do not get overwhelmed in case I've thrown too much information, if you are a beginner, you will lear all of this an a lot more step by step. the general idea of the whole answer is (again): You can aproximate a proper bike to buy, but you can be sure only by actually riding it long enough.
