No major problems.
I first had a 306HDi Meridian from new when they first came out in 1998 a T reg - great car with excellent performance and economy.
Changed it early 2002 for an estate Meridian Hdi
Now bigger fatter alloys, electric windows all round and CD player in dash - still same great performance and economy 53 mpg - pity you can't fit the new engine in it.
Apart from a spell of engine management light coming on due contact fault with clutch cable, the a/con needing re-gassing - both dealer jobs and I changed the discs and pads 'cos its sooo easy to DIY, I cannot fault it - been to the Alps and back utilising the performance envelope (!) and still managed 50mpg over 1800 miles. Will be keeping it for quite a while.
306 HDis actually first appeared on the new Phase III in Spring 1999 on a T-reg. 1998 cars are either 'R' or 'S' plates.
T-reg 306s cover the transition between the Phase II and III, and are usually parts-bin-specials, where they have a mixture of Phase II ('97-'99) and Phase III ('99-'01) trim.