Would it be a bad idea to learn to packraft while using a packraft already outfitted with a spray deck?? I happened to come across a decent deal on a packraft with spray deck, but was a bit nervous to pull the trigger considering the difficulty of exit/entry in moving water. Would it be wiser to begin on an open-boat, and then later retrofit a spray deck?

It's possible that the spray deck is removable or stow-able. The new (2011) models are almost completely removable. Just a small portion of the deck, near the bow, stays on permanently.

Do you know what year it is?

Get it. While it does at least theoretically enhance the chance of entanglement, I think the actual increase in risk is minimal. I've flipped in my 2010 Yak plenty, and have no difficulty wet exiting.

Spray decks are always painfull to get out from. Some worse than others, they are always as bad (assuming quick release harness types) or worse than open cockpits (always a bit worse than open canoes or rafts.) Currents can tangle body parts, paddles, and spray decks dangerously. So, it sort'a depends on your skill in the rafts. No snag clothing is always best. Mostly, you will not be tackeling much beyond a class II anyway. So, if you have it, it helps with rain and waves. Practice a bit in calm water, that will let you know what to expect.

If you can get a raft at the right price go for it. David is correct that the risk is minimal, and you can get that risk close to nil with some practice and adding a wiffle ball or other object to the connection. A deck really helps keep water out, and the rafts' great buoyency makes it easy for water to get in.