Unless the tank is a PST, it's likely that you will not be able to get the plus rating, but that shouldn't stop you from using it. Lots of places will fill that tank to 2500 PSI even without the + rating. The easiest way to find out if it's worth keeping is to look inside with an inspection lamp. If you see a blistered plastic coating or large, deep-looking rust spots, then it might be difficult to get it in good condition. But, if it looks relatively clean inside, maybe with a layer of flash rust, it's a breeze to tumble or brush the tank and get it hydroed. Anyone who has even a modest understanding of steel tanks understands this; unfortunately, that doesn't seem to include the person you spoke to at the dive shop.
The j-valve is almost certainly rebuild-able for a few dollars in parts. If you don't want a j-valve, there are doubtlessly some vintage divers that would trade you a k valve, maybe throwing in a valve rebuild for a small fee. If you're anywhere near vintagedoublehose (check their website) you can probably get some excellent advice and help there.
For shallow shore dives, I use LP72s more than any other tank. They have an excellent combination of buoyancy (neutral), weight (only 26lbs empty!), and capacity (almost that of an AL80) that is very difficult to find with modern tanks.