The drafts are because of bad weatherstrip sealing, not the type/weight of the door.

It seems to me you need someone who makes doors to look at your situation. Reason for that opinion is that the construction of the door/side light assembly and the way it is installed may not support a much heavier door long term.

That means basically finding a custom door maker as the best option and a very experienced door installer as a second option.

You can spend a lot of money on a new door and a couple years late, have more drafts. I built a new house in 1999. My front door is a graphite composite with sidelights. By 2004 I was adding weatherstrip because the drafts were so bad. All my other doors, metal, were leaking air too because the magnetic weatherstrip wasn't long enough on the side jambs. So I would get busy adding weatherstrip. I used the adhesive stuff and it's still on the jamb and does a good job.

You can take your old wood door and dress it up to look new and save a ton of money. go to www.louverandlite.com there you can use the design center to see how it would look and order it right online. and they are all american made products.

Here is a link that might be useful: www.louverandlite.com