I live in Michigan where most of our trails are pretty deep sand and i work the @#%& out of my rm250 at times. it really depends on how you ride and the sand (deep dry sand) if you can ride something w/ a little more power i would recommend it because you might be going through top ends w/ a 125. I always do a suspension tear down a repack all needle bearings and what not after each ride it's a pain sometimes but worth it $$$wise.
Are you going to be shooting hills?
Is the sand deep?
are you an experienced rider?
A little more info and some more people might have some more advice for you
My thought is that you and your bike may me working very hard to get through the sand. and have MCR or someone set your suspension up for YOU riding SAND and it will make a world of difference. hope this is helpful.
I've never rode on dunes just deep sand trails so I would imagine it's a little easier and less dusty w/ it being wet but only that top layer of sand will be harder so there shouldnt be much difference really there is a post here about riding in sand dunes as to tips and helpers it has allot of good info in it
SAND DUNE RIDING TIPS- http://www.dirtrider.net/forums3/showthread.php?t=156360
There is good info in these places read through carefully and try some out before you go (if possible) don't try to remember to much at once you will end up forgetting more than you could remember, out of all the info there maybe take 15% of that and practice it then after you feel comfortable w/ what you just learned try some more new stuff. HAVE FUN thats what it is all about and be safe GOOD LUCK let me know how your ride was
your 125 will be fine in the sand. Mine does just fine in the pits and on dunes. Keep the throttle up and the faster you go the less the work on the bike in a sense because you start to kind of glide on the sand.
I just spent a week in the Oregon dunes with my yz125. it did just fine on almost every large dune. Just keep in the throttle and make sure to check your float before going. i had a problem with my float and was running out of gas 3/4 of the way up, replaced the float and was fine after a short down time. other than that little glitch my 125 had enough power to take me anywhere my friends with larger bikes were going as long as i had a hand full of throttle.