There are a few factors to consider.
1. Are there even kits available for the sort of structures you want? If not, then you have no choice but to build them yourself from scratch. If you are located in the United States, finding 1930's British structure kits may be difficult.
2. Do you have the necessary tools for scratch building your structures? For a start, you'll need an assortment of hobby knives, measuring devices, files, sanding materials, painting supplies, and various types of adhesives. Personally, I never regret spending money on good tools because they will always be useful, but to purchase everything at once will be fairly expensive.
3. If you need only a small number of structures, then buying the tools and materials for scratch-building will probably cost more than buying kits. However, if you build many structures, the cost of tools and materials is spread over more structures, so the per-structure cost goes down.
4. It definitely requires good modeling skill and plenty of practice to build nice structures from scratch. It the same with any skill -- you normally aren't very good when you are a beginner. The more you do it, the better you become.
Here are a few threads from this forum that show what it takes to scratch-build structures.
http://www.modelrailroadforums.com/forum/showthread.php?31194
http://www.modelrailroadforums.com/forum/showthread.php?28155
http://www.modelrailroadforums.com/forum/showthread.php?27015
http://www.modelrailroadforums.com/forum/showthread.php?20155
If you look through the "Structures and Buildings" area, you will find more examples. Some are complex; others are simple. The model railroad magazines often publish "how-to" articles on scratch building. A Google search for
model structure scratch building will find many hours of reading about the topic.
Only you can know if you have the patience to learn the required skills.
- Jeff