Ditto that - 45mpg is easy.
I've had 3 ZR's - a 105 and 2 TD's. The difference between the lower powered petrol and ANY of the diesel models is amazing. 160 performance for 105 money, with running costs that simply no other ZR can rival.
The TD's are great, and you'll easily get a decent Mk1 for half your budget.
The only difference between the 101's and 115's is the map, which you're likely to change with a set of SDi injectors anyway, so save yourself the costs and go for the lower powered one.
110k is no problem at all as far as the engine is concerned, and because of the weight of the TD lump they have 160 suspension, so that should be adequate too.
The brakes aren't exactly confidence inspiring to look at (being just 262mm solid discs up front and drums at the rear), but they work perfectly well on the road, and a full 160 setup (with 282mm vented fronts and 262mm solid rears) is a straight bolt-on.
You can easily find straight alloys instead of the standard hairpins, sideskirts and Monaco interiors to make them look like the better spec'd models from eBay or in scrapyards.
As Peter says, look for evidence of at least the cambelt having been done, and preferably the injection pump timing belt done at the same time, although if that snaps it just stops the engine and has to be re-timed.
Talking of fuel pump timing, it's easy to get wrong, which results in hard starting in cold weather. Not difficult to fix, but a good haggling point.
Also watch out for coolant changes. The water pump is driven off the back of the power steering pump via the alternator belt. If anti-freeze isn't maintained in the right proportions, the freezing coolant can shear the water pump's impeller.
This can be a difficult one to spot because localised boiling can make it look as though coolant is flowing. It's about the only thing that can kill the head-gasket.
The Diesel Forum on mg-rover.org is very good with some really knowledgeable and helpful people. I can't recommend it highly enough - unlike theMGZR.co.uk, which I can't recommend at all!