How to start MX..?

brokies? What? 

who gives a fuck what other people are riding. You're a marketers wet dream kiddo

A $5k 125 is going to stretch your dollar a lot farther than $5k 250f. IMO

The 125 is the best way to get your feet wet with mx.  You have to keep your momentum and keep it on the pipe.  It will help you build your skills until you're ready/able for a 250f. It feels so effortless hopping on a four stroke after ripping a two stroke for awhile. 

-some dummy's opinion

Mite be some deals out there on new 250F's, put a nice down and get some reasonable monthly payments? Follow your Dreams.

Maybe a leftover 2023 rmz 250

find one that works for you and fits your budget.  buying used you will have to work on it either way, I would try and find something that is newer, less then 8 years old. 

Get a yz125.  You can work on it yourself, you’ll learn to ride better faster on it than anything else.  You won’t lose anything selling it either when you move on to the next stage. If you’re concerned about what people think, every true moto head loves a 125.  

I can’t believe no one has said CR500 yet. 
 

really thought, the YZ125 is probably the correct answer.

Heh heh.......

or.....a BRC 500cc 2-stroke....  

Absolutely get a 250f anything with efi it’s forgiving and you will have plenty of power learning, you won’t enjoy riding mx on a 125 unless you’re light weight and want to ring its neck

125 for skill building. It will make you a much better rider if you decide to go to a 4 stroke later in life. 

I voted 125 but depending what dual sport you ride, how long you've been riding for and your size I'd say a 250 2 stroke wouldn't be a bad option. Still cheap to run and maintain and if something goes wrong they are fairly cheap to fix plus you don't have to ride them hard like you do a 125

I haven't ridden a modern 4 stroke MXer.  Do they still have the tendency to nose dive on take off?  Is so I'd start on a 125 2-stroke based on that.

Whatever you buy, make sure you have enough money left over for proper gear. I’m guessing you have an off road helmet since you ride a dual sport, but you’re going to need a good pair of moto boots, knee pads/braces, and a chest protector/roost guard (>$500).

I love 125s, but my first big bike was an RMZ250… I’d argue that it’s the best beginner bike. Not exhaustively slow, not too fast, fairly bulletproof with basic maintenance, and a good segue to other modern 4 strokes. 

What elevation are you riding at?

                Look for a 2-3 year old 4 stroke 250F. Many have been used for trail riding, and while that does not guarantee a trouble free future, it does make it more likely. Assuming that the upkeep has been done. I feel going from an adventure bike to a 125 ( my personal favorite) will be tougher to begin with. Don’t rush, enjoy the buying process.  Good Luck !

Da fuq....

"You won't enjoy riding mx on a 125" 

Something wrong with you, boy.

It takes a while to learn how to ride and race a motorcycle.  Get something less expensive and learn.  Totally not important to beat a 250 right now.  You will know when a 125 starts holding you back, then you can make the switch.  Racing every weekend because your bike is running is way more fun than not riding because you are blown up and out of money.  Have fun.

I definitely do enjoy a 125 but I wouldn’t send someone new to motocross out on a 125 especially if he’s high in elevation 

I'd honestly just recommend buying a new bike, preferably a new YZ125. But there are deals to be had especially for KXs and RMZs. Probably around $7000 OTD. You can finance the rest if you have to. You wouldn't have to worry about putting more money into it.

Yeah. I kind of did a similar thing and I was able to ride a new KTM 350 I got for $7k OTD when I was in school (BYU-I, not too far from you). My sister is in Logan right now going to school now that I think of it lol.