Agreed.125. If you plan on keeping with the db, get a 250f. I bought a 2003 RM125 for 1000 a year ago, put tires on it and cleaned the plastics, sold it for 1500 a few months ago. Value.
HOWEVER, a 125 is a beast and it is not to be taken for granted. It's an on/off switch.
250 four strokes are like riding a honda c50 step through unles you can really ride one, two strokes feel faster ie a a 125 will be more fun but the modern 4 strokes are the way forward in my opinionI would recommend going 4 stroke you get nice consistent power right through. Ive had 2 strokes in the past, its either no power at all or full throttle power at top. If your riding a 600 street bike you can handle a 450 but like guys have mentioned 250 may be better. The new 250f's are all FI too which makes them even more responsive; complete night and day compared to carb. Also 4t's do just as well in trail I've never had a problem. Best bet is find out when a local track is doing a demo day. Manufacturers bring in their whole lineup to test ride!