Depends what you want.
PVR facilities you need an Explora. The 2A is the better one to get rather than the 3A, that is if they are still available.
Not interested in recording content, then the choice is now the 4-, 5-, 6- series HD decoders. The 4 if you can get it still allows for RF distribution. The other two don't.
Then, a LNB replacement is a must. Thee LMX 501 is the more flexible one. The LMX 502Z is the latest version BUT only supports 2 decoders and can't support any other decoders.
Octo LNB; 2 by 2x4 MS; 2 by ES 5-2; Dedicated PSU for LNB, ES's, MS's; HD PVR 4P x 2; Explora 1; DSD 660; FSM
The latest HD one, with a PVR, is the Explora 3A, but you'll get a much better machine by finding the 2A.
Once you've purchased what you want, contact the DStv call centre for direction re doing the swap.
If your purchase is second-hand then you need more info re how to do that paperwork.