I generally agree with the direction your taking. I'm not very familiar Cooper's kits, but my calculations for those ingredients show an OG of 1.035 for a 6 gallon batch. Taking it down to 5 gallons would make it 1.043. 1.043 is much closer to the style guidelines for what you are making. But you are wanting about 5.25% ABV, so you need higher OG. If you make it all up in DME (replacing the Dextrose with DME), and assuming a 5.5 gallon batch, then you need to add 2.5 lbs more DME in addition to the 1.1 lb in the kit; add about 3 lbs for a 6 gallon batch. That should give you an OG of about 1.053. You could get Wheat DME or more Light DME or a combination of both depending on which direction you prefer - hopefully you have a basis for comparison - i.e you've made this kit before. I don't have much to offer on DME - I've had good experience with all I've used and have no real favorites.
The Mr Malty calculator shows about 2 packs for 5.5 gal ale at 1.053. But you can probably get away with 1, especially if you like banana and clove flavors. The Wyeast site (http://www.wyeastlab.com/com_b_yeaststrain_detail.cfm?ID=135
) has good info on this yeast including guidance on controlling phenols and esters, and you'll probably find more on here. I have heard of some wheat yeast as being very active (i.e. needing a blow-off depending on headspace).
Save the dextrose for priming. Priming is best done on a whole-batch basis. For a 5.5 gallon batch you'll need about 5 ounces to hit the middle of this style (2.7 vols)