Ok so currently I have an iPad 2 and a ultraportable thinkpad. I've been using the iPad 2 for 95% of my uses but need something capable for he rest. Was going to get a MacBook pro or air but it honestly would not get used enough to warranty the purchase. I also don't want more than 2 mobile devices. The iPad and my android phone is good enough. But I do want something capable enough to handle anything I may possibly may ever want to do, realistically speaking, nothing too crazy. An iMac could handle it, screen is beautiful, fantastic piece of art for my home office, and of course it will last a very very very long time (7-8 years) On the other hand a MacBook pro offers me the performance while also giving me the option to be mobile with it for if were to decide. iMac + iPad MacBook + iPad Or just stick to buying think pads and combo that baby up with an iPad?
I'd say an iMac, probably one of the basic models. If an iPad can do the majority of your computing, there's no sense spending more on a mobile computer that won't be moved.
Dude...you are all over the board! LOL Everybody's needs are different. My sweet spot is my current arsenal; MB Air, Apple Cinema Display, iPad 2 and iPhone 4.
I Don't think you'll get 7 years out of any computer. I recommend a refurbished iMac to save on cost. They look brand new.
An iMac appears to be appropriate to you. As regards the size of the display, either 21.5" or 27" seems to be OK to you. If you can afford it, you should go with the 27" base model. You can then enjoy greatly the computing at the current time of history.
If you are willing to invest the money into it, the most versatile and ideal solution is to get a MacBook Pro hooked up to a 27" Cinema Display. It would take a huge hunk of cash, but you get all of the advantages of an iMac plus it's ultra portable. Or if you don't want to invest quite so much, you could go with a MacBook Air instead of a Pro.