YZ250F rebuild advice, what does it need?

75-100 hours is about the right time to look into doing the crank. It is a lot cheaper to stay on top of it than wait until she blows.

Checking the valves should be part of your 5 hour maintenance. You should know where you are at shim wise and the state your seats are in just based off of that. You should still disassemble and check for wear while you have it apart.

Do not ever use a wiseco crank, but the pistons are OK. I would still stick with ProX or OEM for the pistons.

For the crank, OEM is the way to go. Either buy the complete crank (will run you about $330), or the rod, bearing, and pin (will run you about $140) and rebuild it. If you do get it rebuilt it is a good idea to get it balanced. A rebuild+balance (with the pin welded where the crank is true) on it will cost you about $200 plus shipping if you send it off, so maybe 390 for a rebuilt crank.
You could also get a pro-x rod kit rather than buying the OEM parts. Pretty much the same and it will only be about $100 instead of $140, and will have the little end bearing too.

I have an 2009 YZ250F with 165 vet hours, here is my routine;
3-5 hrs fresh Rotella T, clean stainless steel oil filter every 2nd oil change
~25 hrs check valve clearance, usually require shimming at ~50 hrs
~50 hrs replace piston and cam chain

At 125 hrs I replaced valves, seals and springs. Guides and sets were in good shape.
Had a piston failure at 165 hrs, expansion ring came loose and went into crankcase. Split the cases and replaced a few bearings and cleaned everything. While it was apart I had the crank pressed apart and inspected, everything was nice and tight, no signs of excessive wear so I had them press it back together and true it etc. I will check it in another 50 hrs when I do my next topend and see how it feels. If she gets loose I'm going to add a stroker to the 290 :-) If the prices were the same as quoted above up here in Canuckia I would have replaced the rod when it was apart.

What does everyone recommend for valves? OEM? Seats get recut, not replaced, right? Also, will I need new springs?

(On a Honda...)

IMO change the air filter as soon as you can during race or practice day, this will reduce the amount of dirt particles that get smashed between valve and seats. Recutting seats would be the first step if they are not sealing. There is some conflicting opinions on lapping the valves, some say the coating on the valves gets worn off during lapping and leads to premature wear... not sure. I would replace springs with valves and seals.

Stainless steel valves are very durable but the increased weight makes it necessary to use heavier springs, these increased weights and forces of valves and springs can increase wear on the valve seats.

Complete Honda heads are surprisingly cheap. Just over $300 on Partszilla.

I have 165 hours on mine. Just did a 100% rebuild... The Cylinder head was done at 150 hours...so I did not need to mess with that. Everything else was a complete rebuild. It is a lot of work...and you need the right tools
*Sent the crank out for rebuild. That was about $175.
*All new bearings. You need a blind puller, oven, press to pull/install new bearings. If you are going to split the cases...REPLACE THE BEARINGS!
*Replaced all circlips and washers in the tranny
*All new seals
*New clutch plates and discs
*All new O-rings
*All new gaskets
*New cam chain
*New Piston, Rings, Pin, Clips (CP CP CP CP!)
*If the cylinder is out of spec...get it re lined. I have found that ever 3-4 upper ends...the cylinder needs to be re-plated

Tools you need:
*Blind bearing puller
*Press
*Feel gauges
*Mics through 4"
*Bore Gauge (resolution to .0001")
*Dial indicator and VBlocks
*Torque wrenches

1. Spray paint black
2. Attach "monster energy" stickers to shrouds.

Seems it doesn't include valves/springs/retainers etc.

hell ya! thats the way to do it

Every single dirtbike on the local listings is rattle can black with monster or metal militia stickers festooned all over every singly area of plastic. If you don't care enough about your bike to not spray paint it then I guarantee you won't change oil and adjust valves.

i would get OEM or DEL WEST valves, and get someone to put MOLDSTAR 90 seats in the head, much better heat transfer, valves and seats last longer.