I've gotten several quotes on replacing my air conditioner, it leaks
R22 after about 2 weeks is empty. Several techs have suggested
replacing both the internal and external units.

I've gotten several quotes around 4000-5000 dollars to replace and
install a new unit.

I see a new unit available online, both indoor and outdoor 4 ton unit
for 1800 dollars delivered....(on ebay its a large organization in FL
selling them new with warrenties Goodman is the model)

My question, how much does it cost to install these units once they are
on the premises?

Is this a job I could do myself? Just solder to the existing pipes
and wire it up to the 240? Then I'd call a tech to fill the system...

Anyone ever install a system to replace a existing one and have any
advise?

I'm VERY good at repairing things from motors to electrical/plumbing.
I have a metal lathe and milling machine, welder etc, so I'm handy and
not afraid to try something

Thanks!