I agree that for what you want to do, a 4 stroke would be best. Don't spend a boatload of money either, 'cause if you are like a lot of us, you are going to want to jump and pop wheelies and you will probably throw it on the ground a few times.
I also suggest a mid bore Honda XR series. You should be able to find a good running 80's or early 90's XR for $1000 or less. I have never spent more than $1200 bucks on a dirtbike, and I have had (since 1983)
73 Yamaha MX100
76 Yamaha YZ125
79 Honda XL175 (same as XR but with speedo and lights)
79 Yamaha YZ250
81 Honda CR125
83 Suzuki RM125
90 Honda CR250....ported, piped, etc.
Get a damn good helmet, and buy it NEW. You must have a full coverage helmet, and spend the extra to make sure it not just DOT approved, but Snell approved as well. HJC makes decent helmets for the money, I have 3 of them. That, goggles, and good moto gloves are MINIMUM, you should also pickup some used boots and padded moto pants. The hip padding will save you some bruising!
Setup like this, and you will have tons of fun, being as safely protected as your ball sack will allow, and you won't lose a whole lot of money if your interest wanes.