You'd be surprised...MB parts for pre-'90 diesels are plentiful and
inexpensive.  Check out the NG alt.auto.mercedes where you'll get very good
support and answers to your questions.  Visit www.thebenzbin.com while
you're at it.

Franko
98 Tahoe 6.5TD 4x4
86 Suburban 6.2NA RWD
81 MB 300D

Inexpensive? Maybe a bicycle? <g>

Is the current engine dead? With diesel costing more than prem.
unleaded, a diesel conversion certainly won't be cost effective. (And
you say cost is an issue...)

I think you should reconsider a diesel swap if money is an issue,
you'll find there will be tons of unexpected stuff that must be done
and these will cost. I thought you were doing it just to 'put in a
diesel' and had funds, so to speak.

Keeping these comments in mind, I think your best bet is to find
another similar model with a diesel. (I don't think you can, diesel
imports in the US are virtually non-existant!) That will give you the
cheapest and easiest swap.

But, from a money standpoint, just go with gas. You'll never pay
enough to pay for the engine, let alone all the other stuff you will
need to do the swap.