Given your predicament (tail-happy Z3 in the winter months - and, let me guess, the inability to transport anything bigger than a shopping bag?) a 90/Defender is the right tool for the job.
Came to the same conclusion myself after the last two winters and only a Merc SLK for transport.
To be frank, based on my experience, unless you're careful when you buy and plan on spending upwards £7k on the vehicle, paying for the tax disc in the window might be the least of your problems. Scan a few of the technical forums here and you can get a flavour of the repairs/challenges/fun you might face.
If you get serious and find something that looks like it fits the bill, I can recommend a good specialist who works out of Chipping Ongar. As well as being capable of complete re-builds (just completed a chassis transplant on my 90) he'll do pre-purchase inspections for a reasonable fee....