An unused bathroom is a great place, especially if you are doing DWC. I ran a grow in a closet right off my master bathroom for many years. Having water and a drain right there sure makes it easier.
I have 3 different T5s and also cannot imagine me vegging indoors with anything else. While it won't be 4 months before you need the HPS, it will be a couple of months, so you do have some time. I also encourage you to shop around. There can be great differences in price between products that are very similar. I also highly recommend getting an air coolable hood--it makes it so much easier to cool the light.
And why not run at least 1 DWC plant your first go round? You can use an individual 5 gal bucket (my personal preference) and put it right along side your soil plant. The nutrients will only need a bit of tweaking to the pH to work for hydro, too. This way if you do have a hydro disaster (no reason to think you will, but it can happen), you still have your soil plant. A lot of people are afraid of hydro when first starting out, but I personally find it easier than organic.