+1 on el camino.
"I've looked around a little, and the CRF 230 seems to be the best bet."
Tough to find one on your budget, though, and they are heavy.
"There are a lot of KX250s around for a good price, but I think that may be a little too much bike."
You are correct.
"Also, would a 150 be too underpowered to consider?"
No, but it will seem underpowered if you get serious about dirt biking later. If you buy one used at a good price you will probably be able to resell it for about what you paid in a year or so having learned a whole lot about riding and about exactly what you want in a bike. In a year, you might want a serious MX bike, a serious harescrambles bike, a serious freestyle bike, a trials bike, no telling what kind of bike, or no bike. The 150 will teach you a lot in the meantime and keep you off your butt some compared to bigger bikes.
"From what I've heard, 4-strokes are easier for a beginner to handle"
Maybe. Certainly a 4T play bike is easier to handle than a 2T MXer. However, some 2Ts (KDX 200/220, Yamaha RT 180, Suzuki RMX 250, etc.) would make good trail bikes.
"as well as easier maintenance."
The 4T PLAY bikes are low maintenance. A liquid cooled 4T MX bike or enduro bike is at least as much and, arguably, more maintenance than a 2T. This is hotly debated on the internet. I am of the opinion that a high performance 4T is more trouble than a high performance 2T, but that is mostly because I can do at least some of the 2T stuff myself, and there are parts in a 4T whose names I do not even know.
"Should I consider 2-strokes?"
Absolutely. There are those, myself included, who think that your dirt biking life really is not complete until you have owned, maintained, and loved a 2T.