Scratchbuilding is not always as inexpensive as some may think, especially if it involves a scale subject with a hull that is of complex shape- e.g. a Type VII. You have to obtain drawings and materials, and it all soon starts to add up.
Subkrafts offer is very generous, and will save you a lot of trouble, but do you have the ability to fit that boat out with a reliable dive system? If you think you do, then invest in a copy of this book-http://shop.traplet.com/product.aspx?c=294
It's a technical, and a tough read, but if you can get through it, you will get a fundamental understanding of what is required to build a reliable working model submarine. It doesn't give specific 'how to's' in construction, but explains concepts like pressure hull strength, hull seals, ballast systems etc.
Do you have access to good tools and equipment? What have you built in the past- it's generally a good idea to have some previous modelling experience, perhaps you have built a model boat or two, or some model aeroplanes. All the skill sets are transferable. If you've never scratchbuilt anything, I would say you're going very advanced with a submarine.
Generally the cheapest routes are either a secondhand hull or choose a subject with a cylindrical hull shape- then the model can be built around a piece of plastic pipe e.g. PVC, ABS, polycarbonate etc.
Plastic pipe is cheap and readily available, that leaves the fore and aft end pieces to construct, which can be constructed using traditional methods, e.g. bread and butter, plank on frame etc. Or using a lost foam process.