Is there a massive difference in performance wrt waves? I don't have an indepth understanding of it and am interested in case I'm following the wrong path.
This is giving me 3 to 4 hours.
For a pic of how they look check here:
http://en.wikipedia.org/wiki/Image:Waveforms.svg
Basically AC power coming from the mains is sine wave (supposed to be at least

) and certain appliances might not perform as they should if they don't run from sine waves (motors being a prime example) but if you're running only switch mode power supplies (those used by computers) then square wave is perfectly fine.
But generally a UPS that provides sine wave is substantially more expensive and of better quality (there are exceptions I'm sure).
The only thing about modifying a UPS is that you have to have a basic understanding of how a DC circuit works so you can identify if the batteries are connected in parallel or series. A multimeter I would say is a must also, just to double check (you get ones for around R40)
I think generally a UPS has a certain standard of quality while inverters are pretty much the wild west, which is why if the inverter doesn't have a seriously improved price I wouldn't risk it, of course that is entirely my opinion.