Atm I ride a "Endorfin Speed III" Roadbike and I'd like to buy a new aluminium wheelset so I can use it as a gravel bike too. I just do not know what wheelset, tire combination fits and would be sufficient.
In my opinion, there are a few pitfalls that make a 2000s alloy road bike hard to turn into a gravel bike, so you need to make some compromises, depending on the type of gravel riding you want to do.
- Tire clearance: I didn't find specs, but since this is a 2000s road frame, I would suspect, fitting anything beyond 25 mm tires will be tough. I'm sure you will find some offroad tires that work with your frame, but it'll still be far off from the usual 30-40 mm found on gravel bikes. Dirt rubbing on your frame is also more likely than on a bike that fits wider tires with enough clearance.
- Rim brakes: These work fine on the road but many (gravel) riders get sore/tired hands from riding without suspension and (hydraulic) disc brakes are easier to accentuate, then. Rim brakes limit tire clearance and might not perform as good in the wet or when brake tracks are covered in mud - but I doubt you would be riding a lot in muddy conditions with the narrow tires you can fit to your Endorfin bike.
- Road gearing: It is just a guess, but a 2000s road bike probably comes with gearing that isn't really suitable for offroad riding (thinking of a 53/39 crankset with an 12-25 cassette on steep gravel roads, yikes!)
However, if you just want to ride smooth gravel without massive inclines, you might be able to fit something like those well-known Panaracer tires and have improved offroad capabilities - if that is worth a second set of wheels, light offroading could probably be done with a standard set of rigid/allround road tires, too.
Perhaps, a used cyclocross bike is a better platform to get into gravel riding - since these are pretty much the race-focused predecessors of today's gravel bikes. Next tier would be entry-level gravel bikes, since you were considering spending some money on a new set of wheels, this might also be an option in the same ballpark and would do the gravel job better than a bike that wasn't meant to be ridden in those conditions in the first place.