Great choice! 

Combining your #1 and #2...

Best wildlife viewing in the Okavango Delta and Chobe/Linyanti coincides with the dry season, June through Oct, when the grass is shorter and wildife congregates around remaining waterholes.  May and Nov can be good too and usually have shoulder season pricing, but there can be more rain.

If you are interested in the Kalahari Desert region, then the wet season is generally better to see wildlife, so Dec through April.  I've gone in Aug and the meerkats that the area is famous for remain very active.

Though I have not gone in the rainy season, I continually hear good things about delta camps even in Feb through April, especially Kwara and Little Kwara. 

This chart helps answer where to go, when to go, and what animals you are likely to see.

http://www.africa-adventure.com/dsp_besttime.html

Botswana is one of the more expensive destinations.  Doing a mobile safari, most of which are very comfortable with excellent guides, can help keep costs down.

Most itineraries combine 2-3 days in the delta, in Linyanti, perhaps Chobe and/or the Kalahari or nearby Makgadikgadi Pans.

My favorite Botswana camp is Duba Plains.  The documentary Relentless Enemies was made there and looked at the relationship between lions and buffalo.  The lion buffalo interactions can be amazing.

Agents:  Keep in mind that Duba is a Wilderness camp and not every agent out there can book Wilderness camps.  Kwara is a Kwando camp.  There are also camps managed by Orient Express, AndBeyond, Footsteps in Africa, and others.

Some agents you may wish to try include:

Africa Adventure in Ft. Lauderdale
AndBeyond in South Africa, with other offices
Eyes on Africa in Chicago
Fish Eagle in Houston
Go2Africa in South Africa

These can all handle Wilderness camps, but AndBeyond is a Wilderness competitor so I don't know if they would.  AndBeyond is the only option where you go direct to the company managing the camps, but going direct is not a money saver.  It's just your preference.  These companies could also help you with mobiles.  You also could try Masson's for mobiles.  I have not used them, but would like to try them.

Good luck.  Botswana will provide you with a wonderful trip!