You can always use a KVM for sharing Keyboard, mouse, monitor. I think some of the newer ones support printers. Will proably need a router for internet sharing, has firewall, and maybe a network printer, or NAS box. Will still need AV & spyware
MS on some of their office products allows it to be installed on 3 computers, will need to check your copy.
If you arn't going for mobility dont go for laptops. The price to performance ratio is MUCH better for a desktop computer
If you want quiet all the time definatly stick with desktops. Laptops may stay quiet when idle but once there is some load on them most tend to sound like hair dryers (especially more powerful ones).
You can get very small form factor cases now days. Having two large towers can be avoided The trick will be finding the balance between size and need. You will need a case big enough for everything you want but that still isn't big/ugly enough to cause nights on the couch
A KVM will be a must. What KVM to buy will depend on what monitor, keyboard, and mouse you have. If your monitor is a large lcd you may want to consider a kvm that has DVI support. If your keyboard and/or mouse are USB you will want a kvm that has USB support.
If you want a boatload of secure storage one of the more inexpensive ways to go is a home NAS box. That would give central storage that is accessible at all times on the network. I haven't had much experience with them myself but I know toms has done some reviews of NAS products from various companies.
If quite a bit of network copying is in your future (such as if you got a NAS box) you will want a gigabit network. A few home routers now days have gigabit switches built in, that would be a good way to go.
Instead of a KVM, why not use VMWare Player and run both machines from the same computer? That way, you both could take advantage of the same hardware and have your own separate configurations. Only cost would be the one computer and two additional licenses for XP. Of course you'd have to build the virtual machines, but that isn't very hard really. Check out the player at www.vmware.com (free).