Well, my ds is no 3, and I had similar age gaps to you! I had 2 dds, but wasn't TOO bothered about having a boy (although ds was a nice surprise)
One thing I would say is that I also worried that I was just getting my life back before I decided to go ahead with no 3, but was surprised by how very much easier it was 3rd time round, and felt I got my life pretty much back far quicker (even though I bf him until 14 mths)- he just seemed to slot in.
We bought an older zafira- it didn't really break the bank, and it has been fine. Think it cost £5000 or therabouts
Went back to work after my 9mths mat leave, and my career is pretty much on an even plain- I'm not dazzling anyone in my 2d a week, but I'm still gaining years of "experience" and continuing to learn and develop.
Financially, it depends what is important to you. We had a lot of baby stuff already, and happily one of my friends had 2 boys then a girl, so we are still swapping clothes! A few of my other friends had boys and were very forthcoming with clothes etc, which we weren't too proud to accept! Ds is the best (and most cheaply!) dressed of all of them!
Don't know how old you are, but I am 36. One of my friends had her 1st at 35, and now has 4 (no twins!)
I do think you are right to consider the implications, but I also think you have to listen to your heart. I always knew that I wouldn't feel "finished" if I didn't have a no 3. Our house is chaotic and crazy, and they all fight and squabble, but I'm still glad we did it! Good luck