Polaris parts are easy to get... and they officially only stopped making parts last summer. To be very honest... Polaris had better parts support than SeaDoo ever has. The down side is that there is no dealer support, so as long as you can do your own work... Do not be scared to buy a Polaris.
With that said... Polaris ski's shouldn't be too expensive. Depending on the trailer, and what model it is... I would say $1000 to $2000 is all.
The 99 and 2000 Polaris skis, with the "Domestic" engine have a few known issues.
1) The "LR" module is weak. It controls the start/stop, power, and voltage regulation. If it hasn't been replaced, it's going to need to be. A new one will run you $115 to $150.
2) on the 99 there was an ignition upgrade. It's a new stator, and CDI. Chances are, it's already been done. BUT... find out. The upgrade kit is getting hard to find, and it is about $200.
3) Some times they have a hard to re-start issue. There is a fix for it... and it's free. If you buy it, I'll show you what to do.
4) The hour meter can be wrong. Every once in a while... when changing the battery... the display will reset. If it's an injected engine... the FITCH computer will have the real time stored.
5) If it's an injected ski... I would pass on it. The injected parts are VERY expensive if you have an issue.
Anyway... I like Polaris. The engines are VERY good if taken care of, and fed the proper oil. Speaking of that... Polaris originally allowed the use of synthetic TC-w3 oils... but just before they shut down... they changed to API-TC oils. Also... the compression should be around 125 psi cold, on a "Domestic" engine. (it will be red and black)
Let us know what it is, and the price.