If the Op's girlfriend has a facebook account suggest joining some of the brand facebook groups.
Post an ad in Thai stating what your looking for state cash only
and when they reply they will want to speak to someone ( give them a phone number of someone that speaks Thai as that can affect pricing and offers.
When it comes to negotiation you tell your girlfriend what your looking for and let her handle all the verbal negotiation, written negotiation and be handled by yourself using translate
you will get replies some with try and change your mind and go for finance, but some will offer a deal, Most of the dealers are already aware of the discount and offers that will be available during the motor show.
4 door pickup annual road tax is by cc example 2.0 engine 2,900 baht 3.0 engine 6,900 baht anything up to 1800cc is 2,100 baht anything over 1800cc is 4 baht per cc
2 door pickups are taxed by weight not cc
Isuzu has a policy of not taking part in sales war ( large discounts)
https://www.longtunman.com/36009
Some dealers will offer up to 250,000 baht discounts on vehicles that were built in November and December 2021 unlikely to offer similar discounts on anything built in 2022
If you look at the various brand websites most of them offer free insurance but when you look at the details its for finance deals only, however I have found they will offer on a cash deal if that becomes a sticking point between deal and no deal.
When it comes to placing an order make sure that you specify a range of dates that the vehicle was built -/+ 3 months from order date and when you request accessories make sure it written on the order form that you want branded accessories