There is no such thing as a P17. The P (as in P14) means "pattern", and was used by the British to denote the year of introduction. The US M1917 rifles are big, heavy, and a pain in the rear to modify for anything (mill off the rear sight, drill/tap, replace bolt handle, convert to cock on open, replace trigger, replace barrel), and are worth more in original condition than modified. I don't see why anyone ever used them for standard head sized cartridges, although they can be made into nice 375 H&H magnums. A Mauser 98, on the other hand, is substantial yet graceful and can even be construed as light.
A Mark X or other commercial Mauser will still run several hundred dollars to turn into a 7X57. As long as you are comfortable with the cost, that is a good option. After all, they come drilled/tapped, side safety/adjustable trigger, and a fairly good stock.
The idea of a modern rifle is a also good one. I recently bought a Remington 700 Classic in 7X57 for a customer, I believe it was from 1987 or so. Nice lines, nice shooter, and very light. You could also look at 7-08 rifles, but it really isn't a 7X57. If you wanted a modern Mauser-style rifle, the Kimber 84 is a nice rifle.