Go for a 54Mbps router such as the Linksys WRT54G and you should have no problems. They are cheap, reliable, widely supported and as a bonus if you like to geek it out there's plenty of good open-source firmware mods out there to play with.
108Mbps and 125Mbps modes are specific to certain types/brands of wireless client and are not fully compatible with each other, so the moment one client doesn't support the correct high speed mode (even though everyone will be able to connect) you effectively drop the whole network back to 54Mbps.
25m is pretty trivial distance for most equipment unless you expect a lot of concrete to get in the way, so you shouldn't need to worry about spending up big on high power gear.
Hope this is of help!