Cheap MTB with future upgrades v/s costly fully ready MTB
The basic issue is that with most any new bike, if you were to add up what it costs to buy all the components and a comparable frame individually, you get a total much higher than the bike's price tag, even without including costs associated with getting those parts installed. Parts bought at retail get a whole different level of markup. That makes it hard to recommend buying a bike with the plan to upgrade it as you go. People usually get better value by buying in at a level of bike that's well-matched to their actual needs.

The other thing with full suspension bikes in particular is that the suspension designs are far from all being created equal, and the cheapest forks and shocks are extremely mediocre to actually ride. There are a lot of bikes out there where you're more buying the idea and look of full suspension than anything else - watch out for those, because no amount of upgrading really makes sense on such bikes.