Excellent choice, it's one of the best upgrades you can do at this point.
Kettle size is going to be determined by batch size & a little bigger is always better. For the kits you won't want anything smaller than 7 gallons. If you can afford bigger, do it, 5 gallon batches in a 7 gallon pot can be pretty messy at times & you'll have to watch for boil overs like a hawk... but it can be done. You could go all the way up to a 15 gallon, such as a converted keg & still do the 5 gallon batches fine. The advantage there is that if/when you decide to upgrade the rest of your equipment, you'll already have a kettle that can handle a 10 gallon all-grain batch. Most manufacturers making kettles specifically for homebrewing suggest you don't fill your kettle more than 75% of capacity. For the pre-boil volume on a 5 gallon batch, that would be a bit over 8 gallons for comparison.
As for burners, I highly recommend this one as a starter burner:
http://www.compactappliance.com/Bayou-Classic-Square-High-Pressure-Single-Propane-Burner-SQ14/SQ14,default,pd.html?mtcpromotion=PLA%3EKitchen_Housewares%3EKitchen_Electrics%3EDeep_Fryers%3ESQ14&src=SHOPDISCOUNT&gclid=CMXm1LbZ6bQCFWrZQgodtWIAMA (note that I'm not recommending the seller, I just clicked on the first google picture that popped up. You may be able to find them at a better price with a little hunting). Again, it'll eliminate spending even more money down the road as it'll handle a *much* larger kettle & you can find them rather inexpensively. There's several other comparable options that would work just as well. I would ignore the BTU ratings for the most part as they're not very accurate. Especially for those jet-burners.