I know of a couple of people who upgraded the 2-rail version to DCS with good results. There is enough space in the locomotive for Proto-2 if you want to go through the installation. Upgrading to TMCC is probably a bit easier. It really comes down to which system you'd rather have in the engine. If you don't have TMCC at all, you'll have to get a Legacy Cab 2L & base or find a Cab 1 and Base (eBay).
With a Proto-2 upgrade, you may or may not be able to consist the locomotive with other Proto-2/Proto-3 diesels depending on the size of the flywheel. You definitely won't be able to do it with TMCC/Legacy.