Hi Mark
It depends on your budget really, some will advise you go for a truck mount if you can afford it, but there are plenty of good portable machines capable of doing an excellent job. Both have pro's and cons - truck mounts may have some access problems while portables generally take longer per job because of your setting up / down time. I'm choosing my words carefully as I don't want to start another Porty vs T/M war!!

. Best advise is to look up a local dealer (not sure who's up your neck of the woods) I got great advice / service and did my training with Cleansmart at Nottingham but perhaps there is a specialist closer to you. As a general rule, the cleaner on the end of the wand is the one that's the difference between a good job or a lousy job, take your time, think it through and then you'll not make any rash decisions. Another thing to bear in mind is the CC trade is more about your marketing skills than what equipment you choose to buy, you will need a good sum of money to market your company, then you need to take the current credit crunch into account.
All food for thought.
I myself use a Powrflite PFX1350 Portable machine, with a wonder wand, but there are many others with similar specs, the scorpion is one that gets a lot of praise on this forum too. both will give you 500PSI and twin 3 stage vacs, buy a decent wand to go with it and you'll not go far wrong.
Paul