I've been taking photographs for a while, and have started to think about moving to full-frame. I'm on a very limited budget, but I currently have a good macro lens and am quite willing to buy something like a EF 50mm f/1.8 to start off with. I could possibly stretch my budget as far as a 70-200 f/4 L or 24-105 f/4 L.
Primarily, I'm interested in taking photos of small wildlife like reptiles, amphibians and insects, but I have a healthy interest in landscapes, birds and street photography.
I currently have an EOS 30D, and I love the way the camera handles, especially with the dual control dials. For this reason, I think shooting with a Rebel would be a significant step down.
However, I have seen 5Ds and occasionally 1Ds Mark IIs on ebay for roughly the same price as an EOS 650D. I'm sure these cameras have better IQ than my 30D, since they are pro-level bodies from around the same time, and full-frame besides. The question is, are these cameras now so old that the image quality from them is a substantial step down from current Rebels?
I realise I'll be giving up continuous shooting speed and some amount of low-light performance, and that I'd also need to get an external flash, but besides these, are there any substantial caveats I need to be aware of before I go ahead and take the plunge?