Lambda sensors are dead easy to fit so long as you have a 22mm spanner. I think the sensors are somwhere up to £90 but i'm not 100% sure you can also grt a universal 4 wire sensor or try a scrapyard as you would be looking at abou £10 there. I have a spare one which I would offer if it wasn't such a common thing to go on these cars!
It could be your
aswell yes but how does your engine run...smoothy or rough? Is it worse when it's hot than cold?