cyclezen

ok, so I'm an old guy, and very non-expert mtb rider (roadie for many yrs/decades), but I can climb pretty well and not kill myself on sketchy descents.
I don't have a 'modern' mtb - as in 9r; but my bike is very serviceable and was a top bike in the Mid 2000s... I recently gave it some love and upgrades, which turned out great and made riding more enjoyable, safer and faster. Made it much closer to what I might get by buying 'new'.
Wheel base will likely be a result of the choice you make in wheel size. 29 being the most common on the higher end bikes, and preferred by many more expert riders.
27.5 being a bit smaller. Generally 27.5 is a bit more manueverable,in tight situations. I upgraded my bike from a 26 front to a 27.5 (I had a good fork which also fit 27.5) and that made a big improvement in handling, especially on difficult singletrack. 29 does even a better job in handling rough terrain.
From what I see, there are quite a few bike models which come with 27.5 wheels in the smaller frame sizes and 29 in the larger sizes of same model.
I have both a Full suspension and a Hardtail, and my preference is well towards the Full Suspension. It handles more predictably in all situations and offers a more comfortable and predictable ride. Full SUspension weighs a bit more than HT, but I don;t really notice it, riding.
Also recently put on a 'dropper seatpost' (on both the FS & HT) - really super addition which adds versatility to riding.
In your case, I highly recommend looking at and trying bikes at your local bike shops. You don;t really save that much buying online (unless you are really clear in what you want) and the cost of disappointment can be much higher if you don;t like what you decide to buy - online.
Try as many bikes as you can. A good quality air suspension fork is well worth the extra over some unknown product.
If you need to decide between frames sizes, I personally would go with the smaller size choice. Read, there's a huge store on knowledge online. Watch videos, ride bikes.
The newer stuff, from known manufacturers, is really very good.
Buying 'previously cherished' bikes is only successful, if you know pretty much what you want and have a chance to see, handle and maybe ride it before buying.
Hope this helps.
Ride On
Yuri
EDIT: the fork on my FS is a FOX with 100mm travel, for me 100mm works fine on quite rough stuff. Going much more travel isn't going to mean much to my kind of riding. If I was buying new bike 100 would be acceptable, 120-140 would be the meat of what might be useable in any situation, for me. There are some very tight singletrack in areas I ride and I find 27.5 handles quite well, not sure about 29, since I've not had the chance to try on these tight sections...not ready for an e-bike just yet...