Your problem is finding them. The SBA classifieds and ebay are the best bets.
Preston Services sell 2nd hand steam stuff
http://prestonservices.co.uk/category/s ... e-engines/
This'll give you some idea
Generally good small singles start at about £1,500 and compounds at £2,500, though the odd ebay bargain turns up. A Stuart 6A compound went for £850 on ebay last year so you never know your luck.
For a Stuart 5A it might well be worth advertising to the steam modelling world to find a builder. Castings are available from Stuart models.
http://www.stuartmodels.com/products/set-of-castings
A set of 5A castings is £775.50 + 20% VAT
There are a few people around, some on this site, who build engines but remember you're paying for highly skilled labour so you need to budget accordingly.
A lot of the companies who used to supply ready made engines have now ceased manufacture, such as Strath Steam in Australia, though some still supply castings such as Tiny Power in the States. That generation of people who learnt machining at school are moving into retirement.
About the only firm I can find who make their own designs to order nowadays is the Swedish company Vinas Engineering (website currently down) who have taken over manufacturing the well respected B2 designs of Bernt Breding. This is a link to a version of their website on the internet archive using translate.
https://translate.google.com/translate? ... t=&act=url
This is the original company.
http://breding.nu/angmaskin/ It's in Swedish but Google translate works well on it. £2,900 including VAT at today's exchange rate for the smallest engine, which is bigger than you require being rated at 5 IHP.
2nd hand boilers rarely come up so you'll probably have to find a design and get one made. The SBA designs overview is OK but not very informative. You have to use approved designs in the UK. Any welding needs to be done by a coded welder. There are water tube designs, such as the twin drum Lune Valley by Glynn Jones (now retired) which don't require welding. Several years ago a new one of those was £5k.
Hope that hasn't blown your budget, but you can see now why a complete second hand boat is sometimes a more viable option.