Hi Michael,
Insurance wise, a Series (88/109) will be the cheapest to insure. I started off with one of these age 18 with one year no claims at £900 odd full comp. Now being 20 and no no claims, my 200tdi 90 is at £800 full comp. Discos and Freebies are quite a lot more expensive, usually double the price of a Series or Defender to insure.
I can only comment on Series and Defenders, I'm sure Discos are the same (maybe Freelanders?) always have something that needs doing to them. This isn't necessarily something urgent, but they're never perfect. You'll always find something else you want for them. It's not a cheap car to start with.
One last thing to bare in mind is the actual driving experience of most Landies. They are by no means quick, or even car like to drive. Life is much slower paced in a Landy, and it suits me down to the ground. If you've got any chance to drive one before you buy, I'd highly reccomend it. My first drive of a Series 3 and I was smitten. Had one on my parents drive 3 days later. Get out there and have a look, see what suits you.
Chris