There will be nothing to market if there aren't anything to market. I suspect it required maybe only a week of development if you're competent. Other than that, it's about gaining critical mass of stuff to download before explosion, I suspect. Marketing can be something like gathering desirable and not illegal stuff to download, curating it, etc and than giving the link to people who want it. That can be done in your spare downtime until you find marketer worth his salt of set up an affiliate program that incentivized people to market for you.
That is very true, it is something that I could start development on by myself but the actual development won't really take all that long even just working on it in spare time. The longer term job on the technical side will be keeping the central server running well and scaling up as needed. On the other side in the beginning the marketing would be getting stuff to download (as you said) and yes an affiliate program of some sort could help there. In the longer term there the issue is going to be providing help/support which is the big part I'm not really ready to take on. I'm already on-call basically 24/7 for my main site, doing sysadmin stuff for another project (that would likely share the same infrastructure at least to start with) won't be a big deal but I'm well aware of how important good customer service is and don't want to jump into something that I'm not prepared to do a good job at.
Main reasons for finding a partner upfront would be 1) that they have some input in the design process (if you want to market to a content distributor that they can theme this download page to match their site then I want to know about that as soon as possible) and 2) so that they are familiar with how it is supposed to work from the beginning and can give feedback if it's not as userfriendly as they want or such. Neither of these is a necessity, I can code it and get a small site going regardless but I am a programmer by trade and know my limitations and something like this will need more than some fancy code to be more than just a small site.
I've been considering some ideas already and am thinking I will put something together regardless, I guess this is more of an invitation for someone else who would be interested in running this sort of thing but may not have the technical skills to do it themselves to join me.