We're unable to make specific product recommendations, so trying to answer generally.
Assuming bikes are priced fairly, a better bike will cost more. An older bike would be somewhat cheaper than a newer bike, with variation because of wear and condition.
You're more limited on choice when buying used. You get to choose between whats available at that time.
Older bikes can work perfectly well - personally I enjoy passing carbon fibre road bikes on my old 80s steel road bike. But that only happens under certain conditions. Given it was a 15 kilogram bike, I never pass people going up a grade unless they're on a BSO MTB.
Ultimately, you're the power source. An older bike tends to sap your overall power a little more than a newer bike, so you have to have a bit more power to be equal to your coworkers on their newer bikes.
Depending on the attitude of your coworkers - they may simply leave you in the dust no matter how good your bike is, or they might be inclusive. Its impossible to predict - some cyclists like to show off to "fresh meat" and some will go out of their way to encourage people. Just ride and enjoy it.
In terms of things to look for, the older the bike the harder it will be to find suitable parts. Anything older than the 80s is likely to be extra difficult.
You need to decide how important Originality will be to you. I personally have put a 2000's era shimano 105 3x9 groupset onto that 1981 Raleigh Arena frame mentioned before. This gave a 30 tooth large cog with a 30 tooth grannie gear, so the bike was able to grind up steep grades, eventually. And it still looked mostly old - it would take a good eye to notice the black cranks and the uprated cassette/derailleur/brake levers which would be partially hidden by feet and hands.