Pot size for BIAB

Hi everyone, I have been lurking for a while on this site so I though I'd better create an account and get posting!

I returned fairly recently from 4 years living in the UK, doing a lot of travelling around the UK and Europe and drinking lots of different beers and I am now trying to get set up for home brewing. Money is tight so I am gradually getting the equipment together for BIAB. I have a 41L aluminuim pot and I am wondering if I will be able to do 23L batches without over-filling the pot. How do I work out how much water to use at the start, and is there a formula for how much water the grain displaces?

Also, I am looking for copper pipe on Trademe and I have seen some 15m rolls of 12.7mmOD pipe, is this a good size for an immersion chiller?

For the thermometer, should I go for a digital one or just one of those long metal ones that are used in the food industry?

Finally, what is the cheapest way to get a fermenter? Can I just get a 30L food-safe bucket with a lid and add my own airlock etc? Or will it not work out much cheaper than buying one that already has all the accesories?

I already have 3 dozen swappa crate bottles so I will be looking for a capper soon as well.

Sorry for all the questions - hopefully I will eventually learn enough to srart helping other learners like me!

Cheers
Patrick