Having tanks of different sizes like that would make setting up the fish room more difficult, especially compared to the neat rack idea first envisaged. It will also mean more work and maintenance, etc. I woiuld just get 2 of the original ons you planned to get if you have room for them in the house (for now) and try 2 different shrimps and 2 different snails and just see how you get on with those, then when they breed and you get enough you can sell those and you at least will have the full process knowledge/experience before commiting fully, and if you decide to do the full shed as before at least the tanks you have will be the right ones for that rack and therefore reuseable!
Havihg said all that if there is someone locally selling their whole fish room (depending whether that has everything you want/need) for £550, that is a great price (they would probably accept £500 and if they are close there woudn't be too much 'moving' involved so you may want to think very carefully on that as that sort of deal/setup may not be around later - SODS law! There will still be a LOT of work though setting it all up, buidling the shed etc etc. Shrimp tanks don't need deep water, usually a foot or so is deep enough, it is the base area that is more important as that is where they spend their time, and the greater the base surface area the more biofilm will grow!
I still think I would start with a couple of tanks though so you can get a little experience first.
One other thing to consider is the water source, so you should get the parameters of your tap water (take some in a jar to a local fish store if you dont want to get all the test kits at this stage). Cherry shrimp can 'normally' be kept in tap water IF it has the right parameters (mine does here luckily). Otherwise you are looking at RO water and mineralisers etc. Being some where that has hard tap water could be costly. If it turns out you need to use RO water you could use a ZEROWATER filter jug if you are just trying out with a couple of tanks, but a full setup will then need the full RO equipment!