It sounds good to me. The problems you mention are minor as long as, as Rodd pointed out, there is indeed no rust problem. The difficult warm start could be any number of things, but it sounds like you are willing and able to get to know the car mechanically and gradually get things into spec. It could be the warm-up thermostat, could be filters (air/fuel) and/or spark plugs that need to be replaced, maybe it's the valve clearances or the ignition timing, or the ignition vacuum/retard characteristics, injection linkage set-up, idle fuel mixture, etc etc. But whatever it is, if the engine, fuel delivery and fuel injection systems are basically ok, then you will get it to run like a charm.
Perhaps most importantly, these cars are engineered so well that, once they ARE in good order, they tend to stay there for many tens of thousands of miles with only regular maintenance, never failing once.
When you check the engine number as Rodd explained, particularly note the first three digits: 127, 129 or 130. The 127 is for the 230 series, 129 for the 250, and 130 for the 280. Also the cylinder head will have a type designation on it, right above the engine number cast in the block.
There has been debate from time to time on the significance of having the original engine or not. I tend to agree with those who hold that it is not important, like Dan Caron (Dr Benz). Although, yes, it may make the car a bit more special with the original engine, and hence does add some $$ to the price, this is in the end still a 'consumable' part just like a starter motor or fuel injection pump. If you look at it that way, people like me who don't really care actually pay less for the same driving experience! You have to be a real expert to know an engine is not original and you have to look very closely under the hood. What are we talking about really.
The 280 (130) engine fits all models, parts are less expensive and very widely available. You can even find a good used engine for less than the cost of new pistons for the 127 engine. So from the viewpoint of the preservation and enjoyment of these cars, having a less-costly option like a 280 engine available, is a good thing. Plus the 280 engine (like the 250) has more main bearings and should be a bit more resilient as a result.
Contacting Will Samples is a very good idea. I bought my engine rebuilding parts from him last summer and he was very friendly, prompt and accurate, and reasonably priced. Will is very knowledgeable on these cars and, altough not very many people know this, he deserves a lot of credit for getting this group started in the first place - it was he who, along with Rodd Masteller, Frank Mallory, Pete Lesler and others, participated very actively in the early days of the Yahoo! W113 forum.
If you do end up buying this car, or any other Pagoda for that matter (and I hope that you do) then having it serviced and checked out by Will the first time can already make a huge difference in the performance etc. of the car, if previously it was serviced by less-knowledgable mechanics. Many times it's little things that can make a big difference.
Good luck, and please let us know how things develop.
Cees Klumper in Amsterdam
'69 white 280 SL automatic