TiVo usually will not allow you to transfer a lifetime subscription from one box to another. You can always call and ask, but they they will almost certainly say no.
The Mini is an extension device. It allows you to watch live TV and recordings from a Roamio on a second TV.
You can either put your current TiVo HD at another TV or sell it, either way. A lifetime HD does have value. You will usually find them going on ebay in the $200-$300 range.
As for the "best place" to buy a TiVo, if you are looking for the cheapest way, it is to buy a discount code off of ebay. If you want to know more about that option you should check out the dedicated "Roamio Deals" thread: