Peter,

What equipment do you currently have? Do you have any microphones and mic pre-amps? For monitoring while recording, you really only need a few things, and lots of vocal processing is not one of those few things. Some EQ (to EQ out your 'head voice' as much as possible, a pair of headphones that you can independently turn off an ear (usually a simple pan control), and a Shure SM 58 is all you need to make great home recordings from an equipment standpoint. A little bit of reverb back in your monitor mix will help you to sing on-pitch (as does the head voice EQ reduction).

More important is an acoustic environment that doesn't have too much room reflections. I think you built a pretty nice acoustic space a couple years ago. Do you have a clothes closet in that space? If so, open the door to that, make sure it's got some clothes hanging in there, and put your mic in front of the clothes. This will usually do way better than the 'portable vocal booth' things that clip on mic stands.

The reason you want a headphone that you can pan, is to listen only through one, and then slip the dead one off and do the 'guide your voice from your mouth to your ear' trick that you see studio singers do. There's a reason for them doing that - it helps them to stay on pitch and not go flat. you want the one you slip off to be dead so that you don't pick up your monitor mix from the phone into the live mic.

-Scott