Are you totally sure LPs are the guitar for you? I'm a fan of buying quality gear, but your friend's argument doesn't hold much water if there's the chance you'll discover you don't like the LP's neck, you want better upper-fret access, or you need a lighter guitar, etc etc. LPs are awesome, but they're not for everyone. Beware the honey-moon period, too ;)
Also, I'm not sure Gibson SGs are necessarily always as much better than Epiphones as the pricetag would indicate. My understanding (going off forum legend) is that you can snag a very good Epi if you take the time to look hard. Of course, there are some outstanding Gibsons around too (a fair few on this very forum), but there're some terrible examples, too. It's probably true that the average Gibson is better than the average Epi, and the best Gibson is better than the best Epi, but I think with your budgetary restraints you'd be better off hunting around for an excellent Epi than shelling out for a Gibson.
But yeah, you can put whatever gauge of strings on whichever guitar you get. I don't know how the stock pickups on Epis or Gibsons go for detuned stuff, but if you buy a cheap, good, Epi you will be better able to afford some Warpigs ;)