The .25 cal is very limited in high BC LR bullet selection. A 10 twist will work for the vast majority of bullets available. That said, there are some bullet makers like Cutting Edge and Matrix that make longer, higher BC bullets for LR shooting. Neither CE or Matrix advertise the longer VLDs or ULDs. You'll have to call them if you're interested and see what they have and what twist is required and how much they are. Berger may come out with a hybrid, who knows? Otherwise the 10 twist will work for all the other bullets.
Barrel length, is a completely personal choice. Longer barrels will increase velocity but they are also more susceptibleto harmonics (whip) which makes them more finicky to develop accurate loads for. The longer the barrel I use, the heavier I make it.
If I built a 25-06, I would build it with a 26" light varmint contour so i could use it as a field carry rifle for varminting, deer and antelope hunting
In my Sendero 25-06 with 26" barrel, I was getting 3400 fps with RL17 pushing 115 gr CT's. So velocity was more than adequate.