If you are looking for an entry level bike, forget trying to build it yourself, as you can't get the economies of scale bike companies can, and whatever you get will be expensive vs a off the shelf bike. Even high end is cheaper to get off the shelf unless you have very specific requirement.

If you did look to build yourself, 3x7 is almost dead even for low end parts, X5 which is the lowest SRAM full groupset is 9/10 speed. although you will probably find it easier to source Shimano, with 2015 Alivio (M400) looking to be the best of the budget groupsets available.

Here is a link to current groupsets, SRAM has X3 as a partial groupset, but only has shifters and front and rear derailleurs, where as all others are complete. Buyer's guide to mountain bike groupsets - BikeRadar