Having bought both the iPhone 1 and the iPad 1, I can say that they were both great to use and own. I sold my iPhone 1 for basically what I paid for it when the iPhone 3g came out. My iPad 1 continued on until the end of 2013, when I traded it in at target for $200. The iPad 1 was going strong as an iMame and all around tablet until it was replaced.
So really, there's no financial risk associated with first-generation apple devices. Rent them, then sell them on eBay. There will be hundreds of thousands of people who will be willing to buy it at a used price, just to see what the fuss is about.
I would say "once upon a time" to that in this day and age Apple products resale value has plummeted lower than ever. I probably could only sell my used iPhone 6 64gb for about 350-400 at this point and it's only around 4 months old.
I agree with Poppy that 1st Gen Apple products have a much shorter life than the 2nd gen follow up, in the iPhone's case 3rd gen as the 3GS was around for a much longer time than the OG and the 3G. It's almost a miracle that the 2nd Gen iPad is still supported in 2015.