Depends on what you want to spend, what you want for fuel mileage, what you consider "adequate" performance, how much electronics you want to deal with, whether or not you're considering using SVO/WVO, etc, as to what model/year you'd be happy with.
Word of warning - if you're not a hands-on person that is capable of doing a LOT of service and maintenance yourself, visits to the mechanic/dealer can get VERY expensive - as in four figure expensive. Same for parts - some categories of parts seem ridiculously cheap, while others can be ridiculously expensive.
OTOH, once you've spent the time and money to go over a used 300D and put it to rights, they can be extremely reliable and kick down suprising fuel economy. The four speed auto trannys from the 80's/early 90's are tough as nails - after 20 years the tranny in my '87 only required $600 for new seals and some tweaks on the control linkages and it's still going strong.
Try
www.mercedessource.com and check out thier initial catalog pages. It's a quick dirty summary of the different models/years and thier high points. Check out the parts websites and the discussion forums, you can get an idea of the more common items that need fixing and what it would end up costing, to get an idea if you're willing to deal with owning an MB.
An '85 and earlier 300D you'd be looking at best high 20's for fuel mileage. An '87 and '90-'94 300D you could expect 30 to 35 normally once everythng is tweaked in.
I've done a few special mods to my '87, and last week before we got laid off it exceeded 40 mpg average for the week. And I do have witnesses that it actually achieved that - 180 miles on 4 gallons.
But this week I've got the dash ripped out to replace all the vacuum pods for the vent flappers to get the A/C working properly again. Like I was hinting at above, owning an MB can be a double-edged sword.
P.S. - try sending oldnavy a PM and asking his opinion, he's had a lot of hands-on with MB diesels over the years, currently owns a '91 and an '05.