Navicular is a diagnosis that a lot of vets are fairly quick to make, even with out x-rays and that is because they are using it as a general term (heel pain). Getting your horse off the heel is going to help remove the concussion and will alleviate some and maybe even all the pain but the poster that mentioned the fact that it may not be a good idea without x-rays due to the founder was correct.
I forgot about the founder. If the coffin bone has rotated then there is still hope but the way the farrier works with the hoof may be different. You really are going to need the x-rays. You could even skip the nerve block if you are certain the pain is in the heel because you know the founder happened so you'll need to check the coffin bone anyway.
X-rays are usually around $10-20 per x-ray and I think for navicular you'll need at least 4 per hoof, hopefully a founder x-ray is the same type of x-ray and you can kill two birds with one stone.
I'm still not sure I agree with the idea of permanent meds. I think with the right farrier, you may be able to manage this at a lower cost then you are thinking (depending on the severety). You will have some days where you'll need pain meds and you'll have days where, if you show, or ride for long periods of time, you'll want to use the pain meds for precaution.
If it turns out there is no issue with the coffin bone and your issue is with the navicular then try the tildren shot at approximately $1200. It is an amazing drug.
And don't beat yourself up over the pre-purchase exam. For $2500, I probably would not have gotten one either.
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"