Yes and no... Browning used to offer the .308 with the BOSS but not anymore. You might be able to find one in new or excellent condition somewhere, if you're patient.
Only Browning knows. My guess is that the 308 is easy to fine tune with loads without it.
400 or so
Cannot be answered unless you include the serial number.
On some models, yes
No.
"Best" is a subjective term that will be based on your pocketbook and what you are using the rifle for.
Check w/ Browning, but I think you are correct- BOSS will interfere with iron sights.
Go to Browning.com and click on Customer Service. You can download or request an owner's manual.
The value of your Browning BAR Mark II safari will be between 325-625 dollars,for a rifle exhibiting between 60%-90% of its original finish and a good bore.
It would have come with a hard plastic buttplate with the word 'BROWNING' on it.
Start with the manual on dissassembly at the link below. Contact Browning from the same website. Repairing a BAR and complete dissassembly is recommended for a Gunsmith only.
Condition is everything as well as age, and where made. Start around 400 and go up to $1200 depending