Hi I have purchased three kitties, I made a point of limiting my budget to £40 worth of Ether. Here is a bit about what I have learnt...
Firstly be warned it took me numerous tries to purchase a kitty I think because the ap is slow, buggy and there are alot of people as well as 'bots' purchasing these kitties. I had read that the younger the generation the better also the speed of the cat (processing and resting time after 'siring') and to look at the kitties 'Cattibutes' these are a list of attributes each kitty has and some are rarer qualities than others. If you want to check how rare the qualities are check out- https://cryptokittydex.com/cattributes.
I 'sired' (mated) two of my virtual cats and another one was born, but each transaction you are charged which can all add up, you need to also keep an eye on how much 'gas' you use with each transaction the more gas the faster the transaction. So if you are buying and possibly competing with other buyers you might want to spend more on gas to rush the sale through but if you are 'siring' and not in a rush you may want to alter the amount of gas it just means the transaction or birth of your new baby kitty may take longer. To check 'gas' rates go to -https://ethgasstation.info/ I didn't realise this when I purchased my first two but by the third I realised you can change the gas rates. So I now have a G3,G4,G5 and G11 kitties, I don't think I will 'sire' any more as you pay for the privilege and it does eventually have an effect on your kitties speed which will ultimately make it less desirable. You may start off with a 'swift' cat but if you 'sire' over and over the speed of your kitty will slow down to 'plodding' or even 'catatonic'. I am just going to hold and see what happens for now, hope this info helps someone else.