Legalities & or Technical problems of building a Game bundle site for charity?

Honestly, I don't think you've got a chance.

You are approaching this from the absolute wrong direction. You're worried about building a site, and expect to spend 2 or 3 thousand hours coding it. This is patently ridiculous. The site is the simplest part of any bundle. It's a simple display with a mass of links and images, with some javascript (likely jQuery) to make it pretty. It uses a minimal database to store user accounts and purchase history, and links to a common payment provider to take care of the actual purchase. If this took 1,000 hours to develop from technical specifications, I would be surprised.

The first thing you must understand is that all the bundle sites are businesses. Yes, they donate to charity rather obviously, but they exist as a for-profit business. Launching a bundle site is launching a new business, no ifs, ands, or buts.

The very first thing you should have in mind is a business plan. Decide up front how you are going to market (in general terms), what you are going to focus on, how you will be different from the other dozen bundle sites out there, etc.

Then you need to worry about getting games - it can take weeks (or more) of negotiation with a publisher to get a game in your bundle. What terms will you offer them? How long will your bundle last? You need to work out with Steam how to get Steam keys for those games. You need to worry about distributing any DRM-free binaries, Steam keys, Origin keys, or Desura keys. You need to think about every single thing a game storefront has to think about, because that's essentially what you're going to be making.

Building a bundle site isn't just something you're going to wake up one day and decide to do, especially if you think the best way to start sorting out the legalities of the situation is to ask on a gaming giveaway site's public forum.