Buying a car is not what it used to be.
Plenty of information is available online now and people spending the best part of 100k are well informed.
To answer your question clinically, yes, it does reduce the value. If you had two identical cars, one with spot welds and one without, the one with factory spot welds is always worth more.
However - this question can never be answered clinically because no two cars are ever the same. It does not mean it's not worth 85k. If the car ticks all the other boxes, it has history, is generally tidy and original than it could still be well worth the asking price.
I am in Australia and recently bought a 1968 280sl. What I learned after looking at 10+ cars is that they all have their flaws, there is something not quite right about every single car I looked at, even the ones closer to 200k. Buying a 280sl is always going to be a compromise, there will always be something that you miss out on and thats different on every car. For example, on my car, it has the factory spot welds on one side but not the other. It is missing the 'fender notches' on both sides and has had a pretty average paint job in the last 20 years. But, it's a good car otherwise, it's manual, it's got a good list of options so overall I'm happy.
Check to see if the car you're looking at has the fender notches. Don't be too surprised if they are not there.
I suggest having the car inspected but a few other things to check:
1. Does the number stamped on the aluminium bonnet match the chassis?
2. Look on the inside of the bootlid, there should be matte black paint and it should have runs in the paint, this is how they are supposed to me
3. Check for the fender notches
4. There are 4 drain holes under the car where pooled water is supposed to drain. If they clog up the car will hold water. Get under the car, look at those drain holes, are they showing signs of rust?
5. Check the head on the motor.. is it stamped 280SE or just 280? What I found is some cars have the correct block and engine number but at some stage the head has been replaced with a lower compression 280SE head because they are cheaper and easier to find
Obviously history is another important one.. how many owners? Where has it been?
I'm no expert but this is what I learned when looking for a Pagoda.. I was initially distressed because no car was ever perfect but I soon learned none of them are!