So you can extrapolate spaceships smaller than SM+5, and I do it all the time. My experience is that the spaceships books in general gives you pretty cheap ships, its just that when the smallest thing you can build in the system is 30 tons, it looks really expensive. Also, if you avoid using rocket motors, nuclear reactors, and the current armor costs drop a lot. You can also double For example, I made a
Modern SM+5 Fishing Trawler that costs $183,000 dollars using spaceships. You may want Pyramid 3/34, which is essential for making spaceships do things when you're not in space. Including making habitats a lot more expansive when they don't need to provide life support.
The trick to extrapolating costs in spaceships is that almost everything across all the books scales in cost with mass. I think the exceptions are gasbags and stealth fields. So you can design a ship at any size, figure its cost at a convenient size, and then multiply up or down based on its mass. I have an article on
scaling spaceship sizes and costs using SM+10 as the base size every time.
One of the main topics on my blog is how to use spaceships to make small vehicles: I call it
Robots as Spaceships. You might find some of those articles useful.
On the other hand, you seem to have already worked out your spaceship stats...