My advice is to start small and work your way up. Heck I STILL do small
It's easy to become over-enthusiastic with this and let one's enthusiasm overwhelm things. Everything you say about rebuilding machine tools is correct, you can take either a tired or inferior machine, and turn it into something that is much better that it was when new in many cases. However I think many people underestimate the amount of work that takes. I recently saw a post where somebody said he was going to make a living out of it. Good luck with that! It's tiring and hard work, and your back will ache like no tomorrow. The bigger the machine the more your back will ache!!! It's ok for people like me, as I can put it aside for a while.
If you're just starting in this, don't underestimate starting on more basic equipment like your reference tools and learning to make them REALLY accurate. That's still the type of work I prefer personally, but to each their own. Don't worry about straight edges and the like, just get started with scraping and the needs will emerge in the fulness of time. There's no point in, for example, buying a 3' straight edge if you discover you either don't like scraping at all, or prefer to work on small things. You will accumulate the appropriate gear when the time comes. You definitely don't need to start with a Biax, and you will be better served by learning how to first hand scrape well. There are many times when a Biax can't do the job.
For a surface plate, 36 x 24" is probably a good size for a small workshop, and you would be able to do a lot with that. As was mentioned, the bigger the better. Having said that, it's probably a moot point for these purposes, but there is indeed such a thing as a plate that's too large. The larger the plate, the less accurate it is to still meet specs. So let's just say, for argument's sake, you had a 10' x 10' plate, but only used 24" of it and never more. That 24" of the large plate is not as accurate as a 24" plate to the same spec. Again, a moot point, and more a metrology concern, but just threw it out there for info.
At this stage I would just suggest to get a plate, make a scraper and have a go. Everything you need to know will emerge as you go along. Be patient and don't worry about making mistakes. We all started by scratching the heck out of a defenceless piece of cast iron previously minding its own business, but if you keep an open mind and try things for yourself to see what works and what doesn't, with a bit of guidance when you get stuck it's not that difficult.