If you really want to start collecting, okay, start buying. If you just want to play, just get a 2600 of your choice and a Harmony cart, put all the ROMs on it, and go to town. This will leave you more money and space to buy some homebrew carts.
I own the majority of the original carts and a good # of the original boxes and instructions, and while I enjoy the feeling of having them, if I were starting today, I'd just get the Harmony cart and be done with it.
I prefer the 6 switch due to all the switches being handy in the front, and it's the system I had back in the day. But there's no reason I can think of to not get a Junior if you prefer it otherwise.