hello omegasnake and let me say well done on chooseing to start off on the road to bonsai bliss
your first tree is a big choice and as keeping it indoors is a huge thing aswell as you will need to put a lot more time into it due to indoor heating (and there no weather indoors at all
anyway a elm would be a good choice for a first tree as thay r verry hardy and forgiveing. On the other hand fukien tea trees can be right little monsters to keep thay r tempermental at best and down right sulky at worst wth a lot of leaf droping and wilting unless kept perfectly.
how much you should pay for a first tress is upto you tottaly. I wouldnt advise speending a huge ammount on a first tree. You can get sum really nice and cheep little trees. On the other hand you could go mad and spend 500 pounds + on a large elm that will look amzeing and you will have verry little work to do on it yourself for a wile.
Or do whot I do trawl the garden centers to find stock plants then spend a few hrs each night wireing triming ect till you have a tree you have made yourself. Wen looking at that tree you have crafted yourself next to the tree you have baught from a suppler yours willl allways seem more sepcial.
o and on the cat issue ive never heard of either elms or tea's being a issue to them so go for you life and enjoy wich either you get =)