Hi & welcome. Run some searches as there are many threads here with buying advice - you are looking for the "R129" SL.
Some general suggestions to get you started.
SL500 is the best all-rounder engine-wise, particularly for a "future classic". The smaller 6-cyl ones are a bit low on power (it's a 2-tonne car), the 600 (V12) is complex and heavy. The SL60 AMG (6 litre V8) is nice but rare and carries a big price premium over the 500.
They are one of the best 'old' Mercedes from the rust point of view. If you get a nice one it's a perfectly reasonable expectation for it to have no rust at all.
There was a big facelift in 1996 and many consider the post-facelift cars to be nicer. Apart from exterior changes (losing the "2-tone" paintwork, orange indicators, etc.) they had a revised interior and got a more modern 5-speed gearbox as standard.
The other facelift was in 1999 when they got oval door mirrors and different tail lights (externally), slight internal changes, and revised engines.
Cars up to the early/mid 90s suffered from wiring loom problems (biodegradable insulation!) so if you get one make sure that has been replaced.
Although very reliable when well looked after they are fairly complex cars and electrical/hydraulic problems (with the hood, for example) can be costly to rectify. Ideally look for a car where everything is working, if you are told something is a simple fix it probably isn't (or they would have done it).
You may see some cars with adaptive damping (ADS). This is a much more robust and reliable suspension system than used on later Mercedes models, but it's still potentially an extra complication on an old car. It wouldn't put me off buying an otherwise perfect car (mine has ADS), but make sure the car drives well, has no warning lights, and the suspension raises and lowers without issues. The most common ADS problem is that the accumulators ("spheres") contain a flexible diaphragm which can eventually split - making the suspension very hard. They are effectively a service item (like standard shock absorbers) so factor in replacement if you look at a car that hasn't had them done yet. Mine lasted 15 years, total cost to have all four replaced was just over £900.
In general look at as many cars as you can. Condition will vary hugely - that will normally be more important than age or mileage.
