questions about a 1973 dodge dart sport?

I will be aquiring a 73 dodge dart sport in january that has new paint, new tires and new exhaust. The engine that will be put in is a 318 v 8 from a dodge satelite. Some of the questions I have are as follows.

1. how reliable where the 318's (the guy building the car said the 318 thats in it now only has 60 thousand miles on it im not sure if i buy that at all)

2. if the body and interior are in good condition is it worth me paying the 3500 bux for the car?

3. Should i increase the power of it by adding headers(comes with a free set. guy building the car doesnt recomend headers says they are a pain)

4. should I just keep everything the way it is and go for the original feel or would the car be worth more if i added some mods to it?

5. How far should i drive the car during the first few weeks to a month of owning it?

also any tips or advice that anyone can offer would be very much apreciate this is a really cool little car and i want to see it running for a good long while. I know they are very hard to come by and im getting a really good deal on the price. It's somethin I want to take very good care of.