What is a 1979 Ford F-150 4x4 Ranger worth

Antique or historical status is based on age, any mods won't change that status as far as plates/registration goes. It's value is subjective, but their are a lot of truck enthusiasts who are into 70's-80's trucks.

They're asking $18,000 for the truck is that a reasonable price?

I'm thinking maybe to high. Any help would be appreciated, thanks.

it is difficult to put a price on modifications - the type, and quality of craftsman ship mean a lot - as well as the value to you as a buyer. modification has little overall market impact, unless there is a huge customer-base as they are often aimed at a specific use. if i put a 5,000 dollar sound system in a 1987 chevette, it doesn't make it worth 5,000 dollars more on the market, but an audiophile that is looking for a chevette (hey, it could happen..) might consider it worth that much more. ultimately, it is worth whatever you think a fair price is.

Could fetch $5000 Mods kill the value of any vehicle. IF it were a one owner garaged original might be worth $10,000 Search around online doesn't take long to learn this.

Without seeing it it's hard to say if that price is high, but to me it seems very high.