15-17 is the sort of price you will pay for an early car in moderate/good condition. Targas at that price would likely have an issue of some kind.
Parts supply from Honda UK is generally quite expensive, however specialists exist here and abroad, that offer a variety of prices and quality of service.
This difficulty and cost should be weighed against the long term cost of ownership, and comparison with rivals. Do you imagine parts and service for a Ferrari 348 will be cheaper? After depreciation is taken into account is a leggy and battered Cayman S a better bet? These cars were built by the cream of Honda's workforce, in a factory especially built for the task, out of aluminium. Relatively few changes were made over the 14 years the NSX was in production because they largely got it right the first time.
Early cars must be pretty close the bottom of the depreciation curve. The limited supply of cars, and the type of buyer they attract mean that good cars will hold their value. As you will so often hear with old performance cars; buy on condition, and buy the best you can afford. If you can live with the lower powered auto these can offer good value.
Good luck, the search can be great fun, and ownership is even better!
1991 Manual NSX Red/Black
1992 Mazda Eunos Roadster V-Spec Black/Tan