Personally, I would use butt fusion fittings and a Macelroy fusion machine. The machines can be rented. I think it costs me $40 or $60 a day to rent the fusion machine. A 2" male adapter with stainless steel threads will cost under $20. I've picked up a used press and facer, just need to buy the heating plate.
You can also use Harco Philmac compression fittings. These plastic fittings are actually quite reasonable. A 2" male adapter will cost $25-50. An equivalent brass fitting will cost 2-3 times this much. I have used both the Harco and the brass fittings.
I did a project a couple of years ago for a church. The specs were for 2" hdpe or 2" copper. About 200 feet long. I used the hdpe and butt fused everything except where I tied into the existing 2" copper pipe at the old well. There, I used a brass copper tube size compression female adapter screwed into a Harco philmac compression male adapter.