Hi Simon
Net profit being 18k and an asking price of 15k is very fair, however there are factors to consider, how long has this business been going? What has the profit/turnover been in previous years (you are looking for a steady customer base). Customers do not have contracts, what you are buying is purely goodwill, there is always a risk with that. The t/o at present isn't far from the VAT threshold, from your post I gather you would like to grow the business, does the £15 already include the VAT if not you need to think seriously about whether the customers would accept the vat increase (which incidently increases back to 17.5 Jan 1st) or would you absorb this. You are already rightly thinking about marking and sales costs, leaflets and paper ads won't cost a great deal, I would budget £500 for first lot but think about the time it will cost you in design etc. Also is the person who owns it now taking a wage which is accounted for within the business or is the profit their income? That may make a huge difference to your decision.
Just a few points off the top of my head but I hope they help.
Fox