Hello Nubbz, that is a very nice setup! You can probably get away with the 5" in those temperatures. With a vented tube your temps will be nowhere near as hot without and you really don't need to pull that much air through it so long as the bulb is enclosed. So, the 5" should be fine for you, especially considering the Fall is coming up and temps will be dropping. I suppose it will be a month or so before you get everything up and running. You will easily be able to grow with that through the Fall & Winter and that would allow you a couple of grows to fine tune everything and save a few more dollars to purchase any add ons that you may want.
I really hate to steer you up in price but honestly, I would rather have you pay 50-100 pounds more right now than to be half way through a grow and realize your plants are not going to make it because you went a little too cheap on the hood. Then you have the seeds, electricity that you have used and a month or two wasted just to have to start over again. If I didn't believe 100% in the air cooled hood I wouldn't recommend it.