Has anyone built a mountain bike from the frame up? If yes do you have any advice or specialty tool I would need and could borrow?
I certainly never have, and don’t have too much advice – but I will say I know it requires very precise alignment and work holding and I don’t know if our welding table is really up to the job. Be careful!
There are some specialty tools. Check Park Tool.
-cassette tools, bottom bracket tool, and pedal wrench. There might be more…
if you mean building the frame, can’t help you.
if “from the frame up” means you’re buying a frame and components separately, my advice is: do what you can do without specialty tools, and have a bike shop do the specialty stuff for you. Getting a headset tool, cassette tool, wheel stand, etc. is only going to be worth it if you plan on doing this a lot.
Meanwhile some things, like a spoke wrench, pedal wrench, and chain tool, are cheap and fall into the category of “basic maintanance” that if you don’t already know how to do yourself you may as well learn, it’ll cost you less to buy those tools than paying a shop to swap out a chain or pedals.