Sorry, but I believe this plan just can’t work.
If you’re really planning on keeping the car for 10 years, even your 2yo won’t fit in the back row after about 5 years.
It’s dubious as to whether such a conversion would be legal; you’ll need to find out.
It’s difficult to even find a manual Gen-F Tourer, because they only sold them for 1-2 years & they weren’t big sellers.
An LPG conversion is going to be another problem; where’s the tank going to go? I guess you could remove the petrol tank, but is that a bit limiting?