There are other shops besides
There are other shops besides evolveelectrics.com and it may be possible to find the adapter plate has already been made/designed for your vehicle and you don't have to have it custom made. kta-ev.com, evparts.com, electricauto.com, etc ..
The price you name is probably in the right ballpark, especially since you're talking about an AC drive rather than DC. The AC drive systems are more expensive.
Make sure to get a BMS for the lithium pack. There's a big pile of .. er... misinformation out there that BMS's are unnecessary or even dangerous. What's dangerous is to believe that line of reasoning.