Sony do their own docks, there are some on their website but not cheap....
I'd definitely check line in specifically as they don't all have it. Also, (and this a personal pet peeve so might not bother you) a lot of iPod docks have the iPod connector exposed, which, if you are not actually using it is a bit unsightly! So might be worth trying to find one which has a discreet connector or one which you can fold away.
Do you have a specific player in mind, it might have docks specifically for it. Failing that, a bluetooth adapter for iPod dock would work, but I think they're normally £15 to £20 so would eat into your budget.
am planning on buying de a mp3 player for his birthday and gp's want to buy him a stereo/docking station thingy. they're not that IT savvy so I've said I'll do some research. the mp3 player is a Bluetooth one so I assume they could get any of the iPod docking ones AMD a Bluetooth adaptor? Any recommendations? budget is likely to be under £50.