Trek 800 rear shifter/derailer help

[titaniumriders]

hello, i have a project i need advice on, i know very little of how this works.
I have an early 90s trek antelope 800 with 3 speed front ( i just replaced with shimano deore lx m570 rapidfire) and 6 speed rear with rear derailer shimano rd m250.

Im trying to replace the right shifter and components relatively cheaply this will be an ebay build. I would like another rapidfire style shifter , no gripshifts.

I dont know how this works im having trouble finding them, do i just get a 7 speed shifter and set the screws and will that work with no problems, or do i have to change the rear gears? If so; Do i go 7 speed or 9 , ive heard rumours the 9 is more work to fit ?


OR... can i get a better rear derailer that will work in conjunction with a new shifter without changing the rear gears?


Sorry if this is unclear, im new to this (first geared bike!!!! but i would greatly appreciate any input .If it helps i just intend to buy the parts put them on adjust them moderately and then have it set up at the bike shop


THANKS FOR YOUR HELP IN ADVANCE

[cyclerUK]

As far as I know there are no 6 speed "rapidfire" shifters.
There are 7 speed rapidfire but these won't match 6 speed even with adjustment.
See CTC website for rear gear info:-
http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3946

You may get a 7 speed to work with 6 but it won't be spot on.(noisy).
I assume you have a 6 speed "screw-on" block and may be able to swap this for a 7 speed block. Then you should have a chance with 7 speed rapidfire.

If you have a screw-on block then you won't be able to fit 8 or 9 speed cassettes without fitting a new rear hub or a wheel.

See Sheldon site for hub info:-
http://www.sheldonbrown.com/free-k7.html