There's really not many upgrades you need for the Promaster other than a new high pressure regulator and a feedneck, in my opinion.
If you want more firing modes, as Nobody said, you can buy the Kila board which is a great upgrade.
But basic maintenance is minimal and parts kits can still be had from www.icdproducts.com
If you need a simple poppet that will work day in and day out, go ahead and get a Promaster. If you're someone who really likes upgrading their equipment and tricking it out, you can still get a Promaster, but you might be disappointed at your options. But for most recreational ballers, the Promaster is a great value for what you get. Don't be put off by the fact that ICD isn't making paintball products anymore. They are still in business and still sell small parts and even provide service for their markers.