Ok, I'll bite. I am not an expert but I have owned a Sea Doo boat for the last seven years. I just bought about a month ago a 2009 Challenger 180 SE to replace the 2009 Speedster 150 that I had previously which I replaced just for something larger.
So much depends on the condition of the boat and how it was used and maintained that I think its meaningless to try an overgeneralize like that. Browse around the 2-stroke forums here and you will find plenty of folks with boats and skis that are 20 years old or older. Look around on the for sale forums and you will find a couple that have been parted out as basket cases after barely ten years.
A boat that is used in salt water will be in much worse condition than one used in freshwater if it was not cared for correctly. Boats that are left in the water will be in worse condition that ones that were trailered or stored on a lift or ramps. Boats that were left in the sun will have the upholstery and other outside surfaces fall apart long before ones that weren't. Needless to say, lack of oil and coolant changes are detrimental to engine health.
To your question about supercharger rebuilding, that is indeed a maintenance item. The boats when sold had a 100 hour supercharger rebuild interval. The newer rebuild kits advertise a 200 hour interval. I assume your mechanic's figure of 150 came as an average of these. A 2009 boat should have the same parts as the 200 hour kits to my knowledge, but that said, it's also ten years old and it hard to be sure of the past maintenance. You probably want to plan on rebuilding it soon. A catastrophic supercharger failure is also generally pretty catastrophic to the engine itself.
Another thing to consider with Sea Doo in particular is they stopped making boats after 2012, so there are no more Sea Doo boat dealers, and no more parts being made except for engine parts that are common with the skis, or that are generic boat parts. Some parts, mostly hull and cosmetic pieces are no longer available at all unless you can track down used ones. Around where I am at least, it is difficult to find a mechanic that will touch them at all, though I imagine that's different in other areas where they may be more common.
If you are going to be dependent on a mechanic for all your maintenance and repairs, Sea Doo might not be the best boat for you, in my opinion. You might want to consider a make that is more common and still in the business and has a dealer network and you can choose from more than one mechanic. If you want a jet boat in particular, this might mean a Yamaha. Think of buying a Sea Doo boat as a bit like buying a Saab car.
Two weeks after I got my Challenger, I got a low oil pressure alert. It ended up just being the oil pressure switch, which cost me less than $40 for the part and $5 for the socket I needed but didn't have. Plus my time, of course. Someone else posted recently they had the same problem and the mechanic wanted $300 for the same thing and they were only even guessing what the problem was. So even small problems can get expensive quickly if you have more than a few and need to take them all to a mechanic.
I would suggest that you look at buying a 10 year old boat about like you would look at buying a 15 year old car. Be prepared to be fixing something or other every year, and the possibility, even if unlikely, that you'll be spending $3,000 or more on something major at any time. Check it out thoroughly and don't buy it without an on-water test. Download the shop manual and look at the maintenance schedule in it. Spend a few hours reading posts in these forums, most of the information you should know is here somewhere.
