Chris' replies, as always, are informative and accurate. But allow me to add my two cents.
Microphones: There are a lot of cheap Chinese microphones out there of variable, if not dubious, quality. In the low-mid price category I have found that an Australian company called Rode is making good microphones at great value.
Recording/Editing Software: +1 on Audacity. I use it for much of the simpler, personal work I do, even though I have access to top recording systems. It's simple to use and very capable (and not weighed down by flash and fluff like much of the comercial software).
Personal recorder: extremely useful (and eye-opening) for recording gigs and rehearsals. After much research I bought the Sony Hi-MD minidisk. It works very well and has a good quality microphone with it. I feel that its more sturdy construction, its higher quality electronics and (much) cheaper media make it a better deal than the flash-card recorders.
Keyboards: It's nicer to play a fully weighted piano-action keyboard, but the biggest drawback is that they are really weighted - physically heavy that is. So if you need any kind of portability then they will be very inconvenient. I bought a Yamaha piano recently for the family and it's got a good action and decent sounds for just over $1000. It's probably more of a peice of furniture than you're after, though.
As for "semi-weighted," this is just sales-speak...a way to get "weighted" into the sales pitch without actually providing it. It's along the same lines as those that advertise "semi-parametric" equalizers.
Computer interface: you shuld look for something that will give you MIDI in from a keyboard so that you can use it as an input device for your scoring software. I'm not hugely impressed with M-Audio gear; for just a little more money you could get something much better by Focusright, TC-Electronics, or even Yamaha.
As for stores, most on-line stores have equally good prices and delivery, but I'll recommend
Sweetwater...the major difference being that you can get an able, informed and intelligent person on the other end of the phone nine times out of ten.
You may find it useful to read
Electronic Musician magazine - despite being stuffed full of glossy ads the editorial and reviews seem reasonably accurate and unbiased. They are on the web
here.
Hope some of this helped.
[Disclaimer: although I'm involved in the professional recording industry, I'm not affiliated with any of the companies mentioned.
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