I love building up bikes! It's one of my hobbies. I found this older Specialized Rockhopper 29er for a really good deal (under $250). My goal was to flip this bike without doing any really expensive upgrades to it. I really wanted to change the drivetrain out. As you can see in the picture (below) it was equipped with a 3x drivetrain. I really hate the look of the older 3x drivetrains with a front derailleur. I had to make a change so I bought a brand new Prime 9 Box4 groupset. (picture below) This is their most inexpensive groupset option. It cost me about $160 bucks.
I got a little excited and I disconnected the front derailleur and made some changes to the front chain rings prior to taking the "before" picture. But you can get the jist of what it looked like before and compare it to the changes I made in the "after" picture.
I have used Prime 9 groupsets before when building up my kids bikes and on previous bike builds. They make quality products at an affordable price. The cheapest offerings from SRAM and Shimano are in the 400 dollar range, up to $2000 dollars. Instead of aiming to get as many gears into a flywheel as possible like SRAM and Shimano's 12-speed options Box components went in the opposite direction. They use 9 or 8 gears within the same width flywheel. It makes for a very simple but efficient drivetrain that will, in theory, have less mechanical issues down the road. On the Box components website https://boxcomponents.com/collections/prime-9/prime9 it touts that their system provides for a straighter chain line, easy set up, the same range with fewer gears and less maintenance over the life of the drivetrain. when you are looking for a cheaper option I am a huge fan of the Box Components Prime 9 systems.
A little touch up paint and a tune up of the bike and this bike build was complete. I have listed it for sale on a few sites. We'll see what response I get but this could be a great beginning bike for a middle or high school rider.