"1. Could I assemble one with standard tools?"

no, not really. a hydraulic press (or such), riveting tools and bucking bars, pressing fixtures, a drill pres...and a variety of 'specialty' tools are required.

but, i should add that i'm a tool 'nut' and highly recommend adding any and all tools to one's tool collection.

"2. What would I have invested after assembly?"

in tools? hard to say...some you can borrow...some can be used for tasks other than gunbuilding. as a guess, i would hazard that $500 would be a fair figure (bearing in mind that the builder makes his own riveting/pressing tooling from scrap steel).

"3. Is it only for personal use or could it be sold in the future?"

the tools can easily be sold off to the next guy needing build tools...although i would suspect you might grow attached to them...heheheh! for example, that $150-300 hydraulic press can be used for many jobs that life throws at the male of the species. remember....women appreciate a handy man!

the assembled rifle must not be 'purposely built for sale'...but CAN be legally sold as long as any profit is reported as income on your taxes.