Ron, if you are thinking 1974 and later perhaps it is the MK111 model you are interested in. I think the MK111 actually showed up in 1975, however any bike after engine#325001 (frame F125001) is a MK111.
Some changes/ that occurred with the MK111 are;
Vernier adjustable isolastics (can be added to earlier bikes, not sure of expense/effort required)
Electric start (needs four pole upgrade to work well)
Disc brake at rear
Crankcases strengthened from earlier 850 type.
In addition there are many little changes that I am learning of. Recently I was told that the MK111 850 was given a stronger crankshaft than the pre
MK111 850s . Also, I was told that there was an upgrade to one or more transmission seals that occurred with the MK111. British /Italian does the necessary machining to include this upgrade whenever they rebuild an earlier transmission.
I opted for a MK111 Interstate as I needed the big fuel tank for range (300 miles) and I saw the MK111 changes as improvements. I wouldn't rule out an earlier bike though. Also, as has been pointed out, some (but not all)of the MK111 changes can be done to the earlier bikes.
All other things being equal, as pointed out the 850s tend to bring a few hundred dollars more than 750s. Also, I think the MK111 850s tend to run a bit higher than the earlier 850s. When I was looking it seemed the Interstate bikes also cost a bit more than the Roadsters, such is the demand for the larger tank/greater range. The Roadster tank looks quite nice but only holds 2.5(or 2.7 depending on which book you read) Imperial Gallons, so you must plan for refuelling at about every 120 miles. This is very problematic on a long trip, especially in the wilds of Canada where fuel is often more than one hundred twenty miles away.
Personally I also like the look of the big Interstate tank and really like the fact that it holds 5.5 Imperial gallons.
My bike gets 60 MPG at 70 MPH, so I know it will still have about one half gallon of reserve after 300 miles.
The roadster would have the same one half gallon reserve at just 120 miles, not far enough for my type of riding.
If I had found a nice Roadster that fit the bill, then I would have purchased an Interstate tank for it. Used Interstate tanks can be had on Ebay, but seem to command quite a price.
Three years ago I paid $4800 US for a very nice 75 MK111 Interestate.
It was ready to ride, although I have done a few minor repairs over the last three years.
I now think that was a fairly low price for it, although prices have risen some in three years
There is a photo of the bike near the bottom of the photos page> (under my name, Glen Breaks)