Well, Epic Necrons involve proxies, for better or worse (there is one Epic unit that can pass as a Necron. It is rare and OOP). As do Tyranids, more often than not (Nids are also OOP).
For Necrons, many people use Dark Realm Miniatures' Andrayadas (see here), and Plasmablast Games' Orbital Combine War Synthetics (see here) can also work well.
For Tyranids, many use DRM's Skryth (see here). Epic Tyranids sometimes show up on ebay, but they're often expensive.
If you want to build a community, you'll likely need more armies than you currently have. Space Marines and Eldar are fairly easy to come by, since both were produced for several editions. I'd recommend watching ebay and picking some up. You can build a company or three of Space Marines (they're common as dirt, seriously) and a decent Eldar force without too much trouble. That would give you up to five different armies to play (assuming you use some Space Marines to make a Chaos force). Make sure to grab a cheap Titan or two, as well - people like Titans. Smaller Titans, like the Reaver and Warhound, are probably best. People here can give you advice on what's a fair price and what's a good purchase.
Reading the rules over and asking questions on the forum will also help, obviously.
Familiarize yourself a bit with the various lists before you make any purchases - maybe play a practice game or two with a friend. Or just ask for some advice on good starter lists here, and use those to learn.If you want to reproduce your 40K armies for the Nids and the Necrons, that's easy enough to do. What do you have of each?