In terms of photo quality, the differences between the IP4xxx and IP5xxx models are insignificant. the IP5000 is of course discontinued, but has the advantage of using un-chipped ink tanks, making compatible tanks cheap and plentiful. The IP4500 uses the same ink but is a chipped tank, making compatibles more costly and less available. The IP4600 and 4700 use a smaller tank and cost more to run. Compatible tanks for the newer models are rare or nonexistent.
None of the Canon models play nice with CISS systems.
If I had access to a new IP5000 I'd grab it, but such is unlikely. The MP/MX multifunction models use the exact same print engine as the IP line, if they use the same tanks as a IP model, they are the same printer.
None of the Canon models can print CDs in the USA. The IP models up to and including IP4500 can be modded to print on discs. The IP5000 can too.