I've got a SuperStor Ultra. SS guts, long warranty. Any indirect should last a long time IF you don't have leaks in the heating path and don't need to add water frequently. Fresh water means oxygen, and that can lead to corrosion. The potable tank is treated for that condition, but the heating side usually isn't.
The things to look at are the temp loss over time (a measure of the quality of the insulation), and first hour heating capacity. An indirect normally doesn't need to be as large as a conventional heater since it usually has a better recovery rate (the boiler is usually larger than a standalone tank heater), and it is better insulated. Also, often the tank is run at say 140-degrees with a tempering valve on the outlet, which makes it look like a larger tank.