a base rail is 12 inches or ~300mm. For the 135, that is more than enough for most purposes. The longer your lens gets, the less that will be sufficient. I'm not sure the total lenght of the bellows, but adding a 6 inch extension get me pretty close to the max extension on the tapered bellows, so that would be in the ballpark of 450mm total extension. Anything beyond that and you'll need special bellows, or two sets of bellows and a middle standard to join them. If you're going to get longer lenses, I'd add the 6in extention as well. Longer than that is only useful for really long lenses or macro and that requires a fair bit to make it all work. For now you're fine.
If the bellows have pinholes, I wouldn't use electrical tape--it doesn't stick terrible well, and leaves a lot of adhesive residue. Book-binding tape is usually better. Some people like liquid electrical tape (basically some black goo), but my issues it it never seems to actually dry for me, so when the bellows have been sitting compressed for a bit, the corners stick together next you try to open it. Good used bellows are cheap, so if they arrive with pinholes, look for a quick and dirty solution to last a month or two, while you buy a new set. Mine came with square bellows, and I wanted the tapered (it seems to fold up better with the tapered bellows), so I bought a set from a japanese seller on eBay, and it was like $65 USD with shipping.
The outside and film depth for 4x5 film holders is unversal, with a few caveats. If you see single sided holders (more common in Europe than here in the US, not sure about the UK) they almost certaily don't fit. Also remember that there were film holders for metric sized film. They had the same outside size, the inside they were adapted to a different sized film. For 4x5, the related metric size was (I believe) 9x12cm. They'll fit a 4x5 camera, but they won't fit 4x5 film, you'll need 9x12 film, so avoid them, unless you want to shoot 9x12. Most of my film holders are Fidelity or Lisco. They're similar enough that I suspect they may have been made in the same factory. There are also some cheap new 4x5 holder on eBay. There was some small discussion on LFF, with some people complaining about the quality, so--caveat emptor.