I had same problem with my Fiat Panda... It was going thru water excessivly, there was mayo in the oilcap and it was over-heating pretty quickly.
I got a rad-sealer (cost me about £10). You pour it into the water tank and run the engine for 10 mins. It seals any holes in the pipes / radiator.
I then got my GF's dad to do an oil change for me, he fitted a new oil filer (this was free to me as he got the bits from work!) and that was problem pretty much solved! So I'd suggest try the cheap options first before you fork out...
Out of interest, my Panda has also had a new set of plugs and plug leads, an air filter and a new fan belt since I bought it in September. Oh, and a new exhaust box... All in all it's cost me about £80. Not bad considering I got the car for £122 off Ebay with 3mths TAX and 9 months MOT!!
It's now needing a new set of rear shocks, a new brake pipe and a look at the handbrake...
But my GF's dad said he'd do the work!