Roy,
Having just gone through a Psuedo-Restoration project myself, I would doubt that you would have to pay as much as you restore the car to it's former glory. The restoration that the Classic Center does is totally Frame-off, and restores the car to what it would have been rolling off the line, to the freshly painted bolt. You can google these projects at the classic center and motoring investments. I chose a different path, and I am thrilled.
I would look through the "vendors" on this site and contact them about the project. These Gentlemen are extremely knowledgeable, and in most cases extremely altruistic in helping bring these cars back to life. Someone who works on many of these cars exclusively will know where the landmines are in the project. The most important considerations for me, was that the car needed a straight and uncompromised frame, manageable pan rust, and decent chrome and trim (including headlight doors). These engines are incredibly robust and can be refurbished for, in most cases, about five thousand dollars in the right hands (make sure you replace all the seals and seall all the bolts into the block, to prevent leaks). The largest expense may be re-doing and milling the heads and valves. Fuel pump, and injection system may be another couple of thousand to get working right. As for the mounts, (subframe, engine, and tranny) those are easy and will greatly improve the driveability of the car. New soft top and seat covers may cost you another $4,000 or so, Getting the dash in shape with new heater controls is time consuming (another couple of grand). The transmissions are built like tanks, so I doubt money needs to be spent there, except for bushings. So, barring any frame issues, you're looking at about $20k to have a reliable road car.
I'm not sure how you came up with $25k for the Mechanical and undercarraige, but I don't know the car. The only pic I see is a rotted lower tail panel, which can be bought for under $300 from K&K, and welded in. The body work is tricky. Alot of unseen rot can drive your costs way up. With welding repairs in a few spots (behind wheel wells, tail panel) a good paint job is going to be $10-12k, assuming you remove and replace much of the trim yourself. One of the costliest parts of the project is getting all the chrome and trim right. The Chromed-brass side moldings, and lower aluminum trim is very expensive to replace, as are bumpers. The other big nut are the headlight doors and tail lights. If yours are shot, you are looking at $4k to replace both. New Coker tires: $1500.
I'm sorry for rambling, but I don't believe that the W113 would cost anyone $75k to put back on the road, unless it was truly rotted to the ground. (I'd guess $20-30k) If you love the car, and want to restore a moment in time in your childhood, or parent's life, you should go for it. If you have $40-50k into it, you'll still be in the black, given today's valuations. But you will have to dig into the folks in this group, and find the help you need. Stay away from the "restoration houses" that restore everything from Benzes to Porsches to Mustangs. The restorations you see done on Cable TV are entertainment, but a total waste of time and money. If you want a museum piece, I'd start with a better car, and spend the money with the classic center or motoring investments or Buds Benz.
I hope you do it.
Mike Mizesko
Columbus, OH