Well you seem to be prepared to drop the money so just go with as much as you can afford.
There is perhaps another option but it would include a little more work on your end. A couple of weeks ago poster Mendicate Bias posted this http://www.neogaf.com/forum/showpost.php?p=23323145&postcount=9097. You could get the 4GB slim and a HDD from a fat 360, either used or new. You could even get a 60GB HDD and have enough room for now. I didn't do the math to see if this is actually cheaper in the long run either so this may all be a waste.
As far as what to pay for Live, very few people I know have ever paid full price, there are deals around the net.
A lot of people seem to think that the play and charge kit is a must have but there is an alternative to this too. This gets you going on the cheap http://www.amazon.com/gp/product/B000XSBVMU/ref=pd_lpo_k2_dp_sr_2?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B000255IUA&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1KN8JZRK3V0993K286Z7 scroll down too to see what you can bundle together for a little over $30. If you go this route for you battery solution then you could also pick up one or a few of these http://www.buy.com/prod/battery-pack-cover-shell-for-xbox-360-game-controller/q/sellerid/23962916/loc/108/216756348.html just to make swapping in a pinch quick and easy.
I LOVE paying for Xbox Live! I also love that my love for it pisses off so many people.
