I started with my CRF 230 Honda and I love it. So durable and reliable (Hondas are known for that) -- I'd advise against any 2 strokes because of the oil/gas combo every time you fill the tank. Such a drag for beginner riders! Get something you don't mind lifting up every time you drop it, some of these cross breed bikes with blinkers, etc. can get pretty heavy out there in the woods and premium brands like KTM or Husky aren't worth the investment if you're learning. Stick with Honda/Yamaha/Suzuki etc. so you can beat the crap out of it -- trust me you will! And your skill level will skyrocket quicker than the roadies as a result
GP Bikes is great for gear, etc. when you're ready. Depending on your height and confidence the cc's don't really matter. Just go with a bike that suits you. I'm currently looking at upgrading to a KTM 250 and they keep saying it's too big a bike for me. Just don't get a used bike, you can't trust what they've been through and the cost tends to not be a big diff anyway.
Good luck! You're going to love it! Ganaraska Forest is where I ride, highly recommend!!!
It's not hard to throw in a bit of pre-mix. Not difficult to measure out a ratio if you passed grade 4 math...
My last bike before this brand new one I have now was a 2 year old used ex race bike... And that thing (an 04 YZ250F) had less problems (I recall none other than a flat tire here and there) than my new one has in the past 6 months of ridden it (2 blown fork seals, blown rear shock and seems to be a bit more delicate in the crashes).
Would I buy used again if I was starting out? Sure would. Just be sure you inspect it really good and take someone with you that knows their stuff. You can also get a vibe off the seller of how it was looked after. Things like what his property looks like when you pull up to the house, the way he keeps his garage organized, what the vehicles he has are like can all be indications of how he looks after his stuff... People that keep their vehicles in good shape, tools perfectly organized, work space clean, tend to also be anal about their toys.
Nothing wrong with going to a KTM now, they are just as popular as the jap brands at the track, and have some nicer things (hydraulic clutch, new models have jap style rear shock linkage, their 450 has e-start that adds almost no weight) and the prices are very similar.
Don't be afraid of the bikes that have been raced. Typically, they are maintained more as they are used for competition and will constantly have someone maintaining them. My bike is washed and gone over after every weekend to make sure it's ready to go for next weekend. Even little things have to be fixed properly. You're also likely to get some trick parts with the deal (suspension is the top mod for an off road bike, before anything that adds power)
I already suggested not starting on a 450, it's way overkill for almost anyone and can actually make some riders slower. I race my 250F against the 450s and the only time they have the advantage is the longer straights, but when it comes to corners I enter faster, turn harder and if you're in the right gear you can make it to the next corner first, and it's easier to fly in the air.
I second the KDX vote actually, designed for the trails and can handle the tracks for a novice pretty good.
As per brand reliability... Yamaha led the 4 stroke motocross wave. The Hondas were known for valve issues earlier on, but in the last few years, the reliability amongst the brands has been pretty much similar.
In the early 2000s, one of the 2 strokes had a bottom end issue, I think it may have been Suzuki but I'm not too sure.
I started on a 1987 Honda CR125 with a blown out rear shock. No "real" bike experience before that, but I figured it out... Wish I had worn proper boots as my ankles STILL give me problems from that.