I would certainly suggest the Magic Trackpad for a Mac - you will be able to specify it as a free option on the iMac at order time (in place of the magic mouse).
Since you are getting iMacs I wouldn't suggest the Time Capsule, instead I would suggest a large external hard drive that you can hook up to one of the iMacs via USB3 (or thunderbolt if you get an exotic one, but it not necessary for a backup drive).
Then you can set Time Machine up to use that local drive on the first iMac, then also share that drive on your network (System Preferences > Sharing > File Sharing, click the plus symbol and add the external disk to the shared list). Then on the other machine, connect to that drive and add it to the login items so that it always connects, then set it as the Time Machine drive. Both machines can then back up to it.
Or, you could just buy two large external USB3 disks and connect one to each iMac - it would probably still be cheaper than the Time Capsule. I use a bus-powered portable drive for my Time Machine volume and it just sits on the foot of my iMac so it's quiet, cool and out of the way, but large external desktop drives are cheaper and can live quite happily behind your iMacs out of sight.
I can recommend the Airport Extreme though - it's an excellent simultaneous dual band (2.4 and 5GHz) router with USB printer sharing and is really easy to set up and control. You may not need this if you already have a decent router though.
I would also get a decent USB hub for your iMac too - Apple's choice to put the USB ports on the back looks nice but can be annoying if you use things like memory sticks etc.