DS. Like I said mate C-Lab Creator/Notator was great for me (it was developed into Logic) and I highly recommend using it. Back in the day it was either this or early Cubase.
There is a great deal on ebay going at the moment for this software (You may want to check that it is in English before you buy!) But it gives you an idea of pricing. The Export is a great little piece of kit - it gives you 3 more MIDI outs for your ST. It's just a little dongle that plugs straight into the back of your ST. I used to use one and it gave me 64 channels of MIDI sequencing in my poor old ST!
http://cgi.ebay.co.uk/Notator-SL-C-L...ayphotohosting
What makes me laugh is the timing was far more rock solid 10+ years ago using my ST than what it is these days!
?100 should get you an 1040ST and a little monitor with your choice of software supplied. Be careful when you buy an ST - make sure you see a sequencer up and running on it. There are a lot of very sick, ill ST's out there these days. Edit - (some also come with an extra meg of memory - very worthwhile).