Number one would have been a bigger engine. I got the 1776 but now have a 2110, big difference but think you have to also get a good builder because some 2110's are not much better than a 1776 because of the builder not putting all the "good" stuff in the engine. By all means get a bigger fuel tank. I got the 13 gallon tank and thankful I did. Leather vs vinyl, don't know if the extra bucks for the leather are worth it, vinyl is better if you are in an area where the interior may be exposed to the elemants. Audio systems are strictly a personal choice. With the top down, forget about hearing it at 60 MPH.
Might save the money and put it in the engine. Look at getting a good engine from somebody like CB or other quality builder.
Bargin with the builders? don't think so but you may get them to throw in some floor mats or something like that.
Good luck, lots of fun doing the spec, read as many of the posts as you are able and ask questions!