The genesis frames seem well regarded Aluminium or steel. I haven’t wridden either
They’re great frames. I’m perhaps a little biased, but do some research on here, you’ll not find many unhappy owners. I had an 853 Steel Altitude, which I eventually swapped for a Ti Altitude. Both are/were superb. The good news is that it’s almost entirely down to the geometry, which the Aluminium framed Core also shares. What Mountain Bike mag loved the Core …
http://www.bikeradar.com/gear/category/bikes/mountain/product/review-genesis-bikes-core-50-11-40746
http://www.bikeradar.com/gear/category/bikes/mountain/product/review-genesis-bikes-core-50-10-36076
In fact, they loved pretty much every Genesis they’ve reviewed! Says something for the company, the design and the geometry… (oh, and the guy who designed em, hangs around on this forum too, though he has moved to another venture iirc, I’ll preserve his anonimity for now! 😉 )
I wouln’t worry to much about up grades and is the frame the best bit etc.
If you’re already worrying about upgrades before you’ve even bought the bike, unless it’s very simple things like tyres and probably a saddle, possibly handlebars and grips, then I’d think about spending more up front on a better equipped bike in the first place. Upgradability (and worthiness of upgrading) of a bike is important, but you shouldn’t be too worried from the off unless it really is a piece of pig iron already draped in XT and expensive trinkets just to look nice.
Firstly my advise would be buy a bike ride and love it. Get the saddle stem length and maybe tyres correct for you. Then just maintain it replacing bits as needed.
Good advice… Spend time getting the forks set up correctly for your weight too, but in general, just get out there and ride the thing. You’ll know within a few rides if you want a different saddle, tyres, or wider bars etc.