Hello Lucky,
Welcome to the Forum.
300,000 miles, Wow.
My brother had a Toyota, 250,000, without any engine work.
Much hope for my Alphie.
They slide doors are fused.
Fuse 2, by the drivers seat.
It also feeds the sonors and ECU. So if they work it can't be that one.
Fuse 15, behind the passenger side glove box (which can be removed very easily).
The fuse box is a pig to get to.
It also feeds ABS and cruise control, so if they operate OK ....
I don't know which fuse, if any, feeds the releasing mechanism.
Ben Hau will know.
The keys.
I bought a transponder key, as you described, from eBay. About £15.
Of course, you will need to get it programmed. Not as easy as it should be.
As they van is a grey import, it is unlikely that a Toyota dealer will get involved.
Mine was programmed by a local locksmith. £80 !
The Alphard uses a different frequency than most vehicles.
Fit for Toyota Alphard 05-09 Remote Car Key Fob with 4D67 Chip 314.3MHz 4 Buttons.
This worked fine for me, but my van is 2006.
I don't know that it is suitable for the pre facelift model you have.
Good Luck.