If you can find a manufactured pan that meets your needs, they can put the drain where they want and still maintain the needed slope. If you build one, you can, too, but that can be problematic. You need 1/4" per foot from the corner to the drain on the longest dimmension. This means if the drain is not centered, that you need a much steeper slope on the shorter distance in order to maintain the proper slope on the longer end as it has to drop the same distance in a shorter length. If you're tiling it, it makes it harder to get a decent looking transition, and means using smaller tile is better.
Check out www.johnbridge.com for tiling help. I'd probably tile the pan I built, and put in a header so I could center the drain properly. ALso, keep in mind that today's code requires a 2" drain for a shower...an older one may have had a 1.5" drain, and you'll have to replace all of it back to where it gets to at least 2".
While you're looking around, check out www.schluter.com and watch the videos of their shower system - Kerdi. A neat way to make a totally waterproof (rather than water resistant) shower, from the floor to the showerhead.