As mentioned I would hold off until the new iMac's are released. This will give you the opportunity to get the latest and greatest OR a cheaper price on a current model.
Any model will likely be fine. The more you upgrade it the better it will be (obviously) it comes down to what you are willing to spend. The only thing I would recommend is if you get a Fusion drive go with the 2 or 3 TB models because they have larger SSD portions than the 1 TB model (better value). I would also go for a 27" model for user upgradable RAM and better CPU / GPU options.