15C is a very good release if you can find it. the DXP plus is easy to upgrade but you need to find the E-proms for a DXP. You do not need to have PRI to tie the two together.
If you do have PRI cards they can be converted to DXNET with one chip change, I did that at Comdial all the time. The price for any phone equipment varies alot. Yes I have gotten whole switches for almost nothing, but if you need a card you should buy it from Sundance because you will have a part that is tested and they will stand by it and you will get support from this board.