First of all, maybe we should take a look at what your budget is for this project--this will help us advise you on the best way to go to stay within your budget. You say only 3 x 5, but a 15 sq ft space is a decent sized space and it will take some money to outfit it right.
FLAT white paint is great as it is cheap, washable, and is a great reflector of light--better than semi-gloss or glossy. I really see no reason to go to the expense of the insulation panels unless you are in an unheated space in the winter, like a garage or shed. You generally have more things to buy than you have money for anyway.
You can use T5s for flowering, but it is going to take a whole lot of them. Lighting requirements are generally figured as lumens per sq ft of space. For vegging you want a minimum of 3000 lumens per sq ft and for flowering a minimum of 5000 lumens per sq ft. Keep in mind these are minimums and most of us run quite a bit more for flowering. Something to keep in mind is your electrical usage. Because T5s emit less lumens per watt than HPS lighting, it is going to take about 50% more watts if you are using T5s. This mean that every single month you are going to be paying 50% more for electricity than if you are using a larger HPS--T5s emit 92 lumens per watt, a 600W HPS 150 lumens per watt. In addition, HPS is going to give you more yield as the light simply works better. Your lighting is directly tied to your yield. IMO, scrimping on light is one of the biggest mistakes you can make when setting up a grow space. Not only does inadequate light produce way less, the bud will be airier and less dense. So to get 5000 lumens per sq ft you would need 75,000 lumens minimum or 15 4' T5ho tubes using 810 watts....or you can run a 600W that puts out 90,000 lumens. That is 15,000 more lumens but 210 less watts...something to keep in mind whn paying your electricity bills.
More things to think about: ventilation You will need a 6" centrifuge type exhaust fan to deal with heat and bring in fresh air. Your plants need a continual supply of fresh air all the time the lights are on.
Type of medium and nutrients. Are you going to go soil, soilless medium or hydro? Are you going to use organic or synthetic nutrients (you must do one or the other as synthetic nutrients and organics do not work together).
Genetics: I encourage you to peruse the seed banks and find something that sounds good to you and that is fairly easy to grow. Decide what type of high you like and look for something that appeals to you. There are inherent problems with using seeds you find in a bag of weed. There are advantages to growing known genetics. I would advise photoperiod plants if you are serious about this, rather than autos.
I also encourage you to read as much as you can. We have a sticky in the
beginner section with some good basic (but older) books on growing. Some of our ideas have changed since then as we understand more about the biology of cannabis. The more you know, the better your chances of having a successful grow. This is a hobby that does have a learning curve. Some people pick it up quicker than others, but cannabis has quite specific needs and wantsa and you will learn with experience to "listen to your plants" and learn what they need to thrive, but that is something comes with experience. So no time like the present to start gaining that knowledge that will help you grow that dank bud you want. Just like home grown veggies, there is nothing like growing your own cannabis. It can get addictive though...