Resizing necks?

I am interested in reloading for 8mm Mauser (not for a while yet though) and have heard that one can resize 30-06 for it. Are there any difficulties and extra doo-dads I need that can cost me a bundle?

Also, I have seen a plethora of different types of powder. How does one know which one is the correct one?

You "can" resize the .30/06 to 8x57, but it is labor intensive. After running the case through the die which requires moving the shoulder back about 1/4", you'll then have to trim the case about 1/4". The necking up is insignificant compared to the moving the neck back.
Far more convienent is to just purchase 100 cases and be done!
Even if MidwayUSA dosen't carry the 8mm brass, it would be far-far easier to neck-up 7x57 cases.
I tried making some .257 Roberts brass from some .30/06. It was a lot of work and resulted in about 40% loss due to mis-formed cases. Also resulting cases had to be neck reamed due to being too thick, but that was the point, to get some thicker necks to see if a result in accuracy resulted. It did. but not enough to justify the excercise. I found another lot of factory brass, and neck reamed for concentricity and got what I was looking for.
Just buy the brass, After 5 reloadings you'll be far ahead !
If you do reform the brass, be sure to trim to the proper lenght, or else hard or impossible chambering will result and possible excessive pressures.
You would probably want a forming die ($25-$40), and definitely a way to trim, a power trimmer would be the way to go $200~, or use a Lee trim set with a hand drill (~$15.00 plus drill). You could possibly get away with out the form die, but the trimmer is an absolute must.
Go to www.midwayusa.com and look for the 8mm brass, if not, them 7mm Mauser brass is only .039" undersized. Simply running it into and out of a regular sizing die will suffice. You will then need to trim and chamfer the brass prior to loading.
Best powders for 8x57 are: IMR-4064, RL-15, any x-4350's, also BL-C2, and H-414.
The good/bad thing about 8x57 is that there are so many good usuable mid-range to mid-slow burning powders that will work well. Basically any powder suitable for .308wcf or .30/06 will suffice.
Another alternative to reforming brass would be to have a gunsmith ream the chamber to 8mm/06.
Good luck.
Buy at least one good reloading manual before buying any loading equipment and spend several evenings studying the manual before commencing.