Hi there,
Worth taking a look at my diesel website aimed mostly at the L series
www.roverdiesel.co.uk
All in all the 200/400 bubble shaped diesels are pretty bullet proof. Usual faults are not serious or expensive to fix, and are usually related to burst boost hoses or electrical gremlins due to wire chafing.
My 600 derv is tweaked and now on 209k miles producing about 25% more power than standard.
Two types of diesel engine fitted to the 200 and 400, the intercooled one and the non intercooled one. The intercooled one has 103bhp, the non intercooled about 84 or somewhere around that. The intercooled one typically will give better fuel economy too so is the one to have if you have a choice. The non intercooled one can be intercooled using parts from a donor car easily enough, and can be tweaked for free.
Anyhow take a look around my site as it lists the usual problems. All in all probably the most reliable engine rover have ever built. It was even used in the honda civics and accords of the day too. Handily honda also give away their dealer manuals on the website meaning you can download it and use it for the 200/400/600 too.
Any questions then just ask.