The metal used in the Rhodes tines changed sometime in 1974. The MK I Stage-73 and 88 models, as well as the suitcase models, were the most popular models prior to and after '74 (the Rhodes 54, some of the student model Rhodes, and the Mk II pianos were produced later in the 1970's. ALL Rhodes have a stiff action, generally, although you can use silicon spray or teflon powder to loosen it up a bit. The older pianos (pre 1974) generally sound a little better and have more 'bite' to them, but are also subject to tines breaking easier than the models produced after '74 because of the metal used (as previously stated). Also, post '74 pianos say 'Rhodes' on the nameplate, and ones before that said 'Fender Rhodes'.
I have owned about 10 or 12 Rhodes over the years. I currently have a very unique Stage-73 piano. It was produced in late '74, after they changed the way the tines were made, but for some reason it has a 'Fender Rhodes' badge on the name rail. I was told that they were using the old stock up at the time and just stuck that name plate on it. Fine with me, since I think the older logo looks cooler.
I would probably try to find a mid-to-late '70's Stage-73. It was the most common model ever made, and usually the easiest to find. Try looking around at pawnshops or small local music stores. I have found them for as cheap as $50, but I have seen them sell on Ebay for rediculously high prices. Good luck!