First off, I'm not a tech and no nothing about 104 engines other than what I've heard. More on that in a bit.
Sounds like the PO had service through 60K. He/she may have skipped the 90K because they anticiapted selling the car in or around that mileage? I'm not sure that missing the 90K service is grounds for walking away given the condtion of the car. Hard to say. One pro who posts here recently said more or less that a PPI may prove nothing. No way to see everything ahead of time.
Wiring harness-evaporator - won't show up in maint. book. I've heard it said that you can get the VIN and via the WEB, find out who owned the car before. Not sure about this. PO would know about harness/evap/
102K looking better than 70K - I have an '87 Accord with 255,000 miles on it. The interior looks as good as it did the day I bought it 15 yrs. ago. Mileage/time mean less than what the owner did to preserve it.
A local tech I use(10 yrs. MB dealership) told me that the M104 head gasket issue was cleared up around '97. Just passing this on. Pro techs here can certainly verify this. I have seen many posts here and at other MB web sites regarding M104 head gaskets. There seems to be a problem in this area.
Head gasket-evap-wire harness - if you can't verify that these have been improved, you might have room for price negotiating. Then again, it's supply and demand. People love pretty things. Someone else who hasn't a clue what a wiring harness/evap/head gasket are may come in and scoop it up before noon today.
My 2 cents - likely worth less.