Here's hoping everything is above board and legit.
A semi-auto Thompson (Auto Ordnance) is still a Thompson.
As for the SP1...if your friend wants to sell it to you for $300 and you would like an AR15 in that configuration to shoot and enjoy you should go for it as his price is very good. Like 1970's-80's good.
Anything on an AR can be switched around and reconfigured but if it were me (and it's not
) I would leave the SP1 in original configuration and enjoy it as is, being happy with the purchase price.
While you are enjoying it, you could build up another AR over time and learn all the intimate details of the weapons and get one configured exactly how you want it.