£400-500 is nothing compared to some MOTs, my last MOT for the Clio cost me £200 to get it through and that was to replace a brake pipe that was made worse by the fact I tried to use it without the proper tools.
If it helps influence your decision, my diesel espace cost me £1870 before the MOT and another £120 on top with the MOT. But look around to see what else is about to go wrong before deciding, it might just be £500 and thats it for a year, or £500 and about a week down the line, £1500, it depends.
And if you have a DCi engine with the rocker cover fault, I have a spare gasket here I'm trying to get rid of
