For billing software I will use WHMCS. For support I have an outsource team of computer tech. For website and design I have a budget of hundreds (which is quite sufficient I suppose) I'll get either a US or a UK Quad Core server.
If I assume that you are talking about online clients, then you have to take a strong online marketing strategy to promote your site so that your site can appear in the top of various search engine result pages and then the visitors are looking for the niche service can easily find you by using some popular search terms relevant to your business.
Direct Mail (yes, physical mail!) It works wonders if you target small businesses. Larger business, the mail will usually get thrown out before it gets in front of the decision maker.