2.6 is a 3.0 - only difference is the cylinders have less volume. So to answer your question, yes the 2.6 m103 engines are likely to last just as long as the 3.0s. Depends on how it has been treated, naturally. If anything, since the 2.6 makes less power than the 3.0, the internals and the rest of the drivetrain are under less stress so theoretically they should last even longer. Less power usually equals marginally better reliability and longevity. But only marginally at best, and only theoretically.
For $1500 I'd buy it right up. You never said anything about what it looked like however. I'd only buy old MBs that don't look their age. If it's got a sun-baked cracked trashed ragged-out faded interior I'd probably pass. If it needed paint I'd pass. If it smelled like pee I'd pass. But for 1500 bones I'd have to think long and hard about it, might make a great MB beataround. The 190Es are very nimble little cars (if the suspension is still tight that is).

Also, ain't nuthin' more expensive than a cheap Mercedes-Benz.

__________________
'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)