I've got KX & Mx both, very similar cameras in that they're mechanical shutters. The MX is very light & compact and would be a good choice for someone who doesn't want a heavier camera. If you're going to do nature stuff it will also take a winder. The KX won't unless you want to spend collector $$.
About the only advantage in the KX is the depth of field preview & mirror lockup. Both features are great if you do macro work. If you are doing work that doesn't give you the time to check DOF then it's immaterial.
There's really no reason to go to the older cameras like spotmatics or even the auto series (ME/etc) but they can be had for peanuts. I think you should be able to pick up either KX or MX body for less than $100 on ebay. I just got my KX for $49 + shipping
If you want to use screw mount lenses on the KX or MX there's an adapter available for that purpose. That would give you long lenses on the cheap.