The "good" kit gets you the box and weather stripping for what I consider to be a reasonable price (remember, you'll have some shipping to cover as well). If you're good at building, you can probably build a box for that price, but getting the weatherstripping too, probably not. However, if you're good at building and already have weather stripping around, you could do that yourself. :-)
If you can get the reflective shield and insulation for less than the incremental cost, then the bigger options may not be necessary. Based on my last shopping trip, the reflective shield "better" would be worth getting included, but you'd have to check current prices (don't forget to include shipping if looking online!). The upgrade to "best" looks like your most likely bet to beat their price. What I've found is that insulation is normally the cheap part and that it's all the other items that get expensive on trying to piece together solutions. If you already have some leftover insulation around, which many people do, you could use that. Although, their insulation may be better than just stuffing in part of a regular roll, since they do claim R-50 completed.
Certainly it's possible to do any or all of the above yourself, and it would vary based on your skills, materials already on hand, etc. Me, even though I'm somewhat handy, I'd buy the "best" kit since I expect the kit to work together well. I'm willing to spend an extra $40 to make sure I get a workable kit right away vs. finding out my insulation isn't a good fit, having to source that (possibly paying shipping again or having to go shopping locally), all while having heat continue to leak out. OTOH, if you really don't have $40/your hair is on fire, you may decide that you really need to do more of it yourself. It's all very conditional, I really can't tell you what to do. :-)
