External/internal door??

Hi everyone! Can anyone help -we are knocking down some walls to open up our existing utility room, kitchen and dining room and make a large living dining kitchen. Currently there is an old glazed wooden door in the utility leading to a tiny enclosed back porch which in turn has doors leading to the garage and the garden. The door from the porch to the garage is a lightweight ply type door and the door to the outside is uPVC. The porch is obviously very cold due to draught from garage and is uninsulated.

The problem is that we can't decide what kind of door to replace the door from the utility to the porch. It will be right in the middle of our dining kitchen and will need to open in the way due to the small porch. I feel like a PVC (external type) door would be overkill and wouldn't look right. I had hoped to reuse one of the internal doors we will have left over as they look nice and will fit with the decor but now I'm concerned that the room will be really cold and draughty.

Also, we will have a building inspector coming from building control to see a beam and french doors we are installing and I have a slight concern that they might take issue with it, even though it's not part of our building regs application...?

Sorry for long post, thanks for reading. Would really appreciate any bright ideas/advice for what kind of door we could use! (cost is also a factor!)