I now have 2 E28 M5s and am happy to share.
I love what a small world it is. I bid against several folks in this thread on BAT for various M5s. Got winning bid on one (88 M5's car, I think), but it was RNM (Hard to bid against myself....). My goal was to get 2 M5s (one to rebuild and the other as reference and to drive). I'm happy to say I was able to do this and now have 2 E28 M5s. Both were once BAT cars so you can take a look at them there if you want. I call them the twins as I also have an E39 M5, as well as several other BMWs and a bunch of other cars, so it can get pretty confusing. Anyway, here's the scoop. When I buy a "classic" car to restore, I plan to spend at least $10k on it and usually more like $15k, sometimes more, depending on the car. When the restoration is done, I like to keep the reference car but that depends on the cars, the market, and what I want to restore next. I just enjoy restoring cars and would rather drive and have fun with my investments than have to log into Fidelity to see them on a screen. I am not a dealer. I do not do this to make ton's of money, but I try to do it so I don't lose money. The trick, or more accurately the risk, is in picking the right car(s). And I always try to set realistic expectations when buying and when selling a car.
For the M5s, the first one I bought was the 7/30/18 BAT car that did not make reserve. I'll call this the bottom-up car as most items under the car had been done. This car is nearly rust free, has original sheet metal except for the hood, has new seats, has the original hydraulic rear suspension and has all new bushings, bearings and suspension components. It also has a lot of records. But it needs a lot. It needs carpet and some interior/exterior trim, some door/trunk/window gaskets, it needs a replacement dash, it is missing ALL of the M5 specific NLA trunk panels (fortunately, having another car makes it easier to fabricate NLA parts...), and it needs a full paint respray. The non-OEM radio faceplate is missing but that doesn't matter because the amp and speakers are not hooked up. The original wheels have a peeling issue and will need to be refinished, but at least they are true. (I have an extra set of BBS RS 090 wheels that I might leave on the car...) A/C is said to be R134a, but it doesn't work and I'm going to have to open it up anyway, so it will be R134a and very cold when I'm done. The exhaust is a frankenstien of parts, some of which say TOYOTA! The car runs well, shifts well and drives like it is on rails, but a compression/leakdown test after I purchased it shows one marginal cylinder so the engine will require at least a valve job. For me, that probably translates into new rings, seats, guides, bearings and seals, or a complete rebuild, top and bottom. That's OK, though because at 224k miles, a rebuild is due. There are many other small things wrong with the car (seat switches, mirrors, a fusebox wiring issue) but most are easy for me to fix. The RNM price was $16k. I paid about $19k for it. I will have a lot of sweat equity in the car, plus at least another $20k by the time I'm done with it. But it will be almost as new. I had the car inspected before I bought it, of course, but they can't see everything. Fortunately, I got exactly what I paid for and pretty much what I expected.
I may have been much luckier on the 2nd (reference) car but I won't know for sure until later this week. I reached out on this BB with a WTB and got a couple of responses. One car was listed on BAT back in 2015 and was actually the first E28 M5 BAT ever listed. I had the car inspected. The images and report came back pretty much identical to the BAT auction detail from 2015 and the car only had a few thousand more miles on it since then. It looked really, really good as that BAT seller had done a top-down refresh on the car. Paint, interior and engine compartment are excellent. Mostly original sheet metal (I think one fender might be a replacement), zero rust. All gooahjez and switches work including the sunroof. BBS RS 090 wheels with nearly new (but 2014 date code) tires. All very good. But the car has no air (is still R12) and an E36 radio that is hooked up but doesn't work. I'll be running a compression/leakdown test tomorrow. There is a small "bounce" in the drivetrain, which could be a guibo, mounts or something else, and I hear a slight whine when driving it that I think might be wheel bearings. The exhaust is original but there is a heat shield rattle at about 1100 RPM. There is a small shake in the engine at idle, which to me means there may be a valve seat issue or an injector or two, but I will know for sure tomorrow. (A perfectly running, fuel injected BMW 6 should have zero shake.) This car also has roughly 225k miles, so a valve job would be due anyway if it hasn't been done recently. I do not yet have all the maintenance history paperwork on this car as I just took delivery of it on Thursday. I paid roughly $25k ($10k more than the 2015 BAT price, but it sold once since then) for this car and it was worth every penny. I'm prepared to put some sweat equity and another $10-15k into it to make it pretty much perfect.
So there you have it. Two cars, two prices and a nearly complete rundown of the issues those cars have. Somebody else said a sub $20k M5 is a project M5. I'll adjust that to say that will get you a running project M5. I'll also say that one way or another, $40k-50k is a reasonable number to spend on a very nice E28 M5 if you are starting today. You either spend that gradually as you restore the car yourself, or you spend it all up front, trusting that the critical work to restore the car was actually and correctly done. Depends a lot on how handy you are and how patient you are. In general, if you see rust or any hint of rust (even if you can weld), at least pause, but probably run. Most other things can be addressed and, even if the parts are NLA, can probably still be resolved. The single greatest thing about BMW, besides the cars, is the community and everyone's willingness to help. I've owned a lot of other brands of cars, but no group can keep a car on the road like the BMW owners group can.