Hey buddy. Welcome to the forum.
I built a pretty decent bike off a full front suspension bike. This is what I did.
Get a good large framed bike, with a full front suspension. I recommend something with disk brakes (mine has a front and rear disk brake setup). This bike had 3 front gears and 7 rear gears. I carefully chose a bike with a thumb-click gear change, versus a twist changer. I did that because I wanted to make sure I can change it easily on the fly. And in my personal opinion, it's easier for me.
I got a china girl engine, so it's a simple 66cc two stroke. Added a jackshaft kit (so the engine drives a shaft that is connected to the pedals. I can then change gears using the rear 7 speed setup vs just the single fixed gear.
After that you can upgrade whatever you want. I added a centrifugal clutch and a pull start.
Having this setup allows me to ride anywhere I want. I can ride in the mud, up and down hills, make jumps, and basically do anything I want. The disk brakes really help and make me feel confident. Plus changing gears make it effortless to climb.
Make sure you get a decently large frame. I did a lot of research before I bought my bike of choice. I did 3 months research, then purchased the bike and kit.
Here is a link to my build. my build thread
Hope this helps, buddy. Either way, enjoy it. By the way, this is my second build. I recommend you buy a cheaper bike first to play with, then move up to a better bike to build.