And, after a comma - acceptable or not?

It's not standard punctuation, no. In standard punctuation of a list divided by commas, you leave out the comma and put 'and' as the final item, so it should be like this:

'I really was hoping to buy a cat, but there is so much expense and then i have to consider where to put the darn thing.'

The word 'and' is a conjunction i.e. it connects one part of the text with another- it connects the rest of the list with the final part of the list.

In lists, no, it's not usually considered acceptable.

e.g. you'd say 'Red, green and blue' not 'Red, green, and blue'. Both are technically correct, but the former is far more acceptable.

Lists where some items have their own commas or are particularly long should be separated by semi-colons.

e.g. 'I took several things with me: a pen, which I had received from my late grandfather; a small leather satchel filled with torn up pieces of paper; a large book, complete with bookmark; and a bottle of water.'

This prevents confusion.

In your example, eliminating the comma would be okay, but equally there is a kind of a natural pause there, and since both 'items' in your list of reasons are fairly long, so you could get away with not using it.

There's no definite rule on it, I'm afraid, you just use your own initiative.

To be honest, reading those examples I prefer the second, seems easier to read IMO.

With a list like that, you would definitely not use a serial comma whether or not you 'prefer' it, it is widely held to be incorrect ;p

It does remove ambiguity though.

In that example, there is no ambiguity. The Oxford Style and Writing guide states that it should not be used unless there is some possible confusion about the meaning.

Red, green and blue is definitely just a list of three colours. 'Red', 'green and blue' as two separate items makes no sense.

If you had something like:

'The rooms in my house are decorated in many different colours - red, yellow and blue, orange, green and yellow, and magnolia and purple.'

then you need the comma.

If you're going to have it in the second case, surely there's no harm in having it in the first case in order to maintain consistency?

What do you mean by "standard"? The Oxford comma is fairly commonplace I thought.