Protip - check your local craigslist or classifieds for fish tanks. I scored a 125 and a 55 gallon for only $100. Tons of people practically give them away when they are moving.
I miss my lemon oscar :(
I've never used a script with NMZ but I've used an autoclicker to flick the rapid heal on multiple accounts and haven't gotten one banned. It's definitely not as AFK as botting it outright, but IMO it might be a lil less risky
OSRS doesn't utilize the GPU.
CPU and memory amount matter more.
Realistically, if you aren't planning to use the computer for anything else (productivity, other gaming, etc) it will probably be more cost effective to just rent a server.