How easy is it to change the PSU?
It depends. Sorry but thats the answer I have to give. If its a slimline PC with a small non standard case then it can be differcult to source a replacement. If its a standard power supply its simple, really simple.
Since its AGP theres still some choice I assume you want a PSU and a graphics card for your £90-120 in which case perhaps £50 for a power supply and get something like a 7600GS for graphics click here about £50. Its what I have, its not state of the art but is a reasonable budget card, there are better cards of course but they cost more and with your processor being a 3ghz I don't see much point in spending a lot more on a card.
Back to the power supply. Open the case and measure it, the size of the power supply case will tell you whether it is a standard power supply are not. I'm sure if you measure it and post back someone will tell you if its an unusual size or not.
I've had a quick look at the manual and it certainly looks like a normal sized case so unless anyone knows better it should take a standard power supply.