Does the web host the site will be on support one or more of the following:
If it does you can get dozens of scripts pre-written to embed into your site that will run an effective keyword search of the site content.
This will be on a keyword basis; if you want to implement a reliable product stock search for available car makes, models, year of registration and whatnot you may need to look into pulling this from an online database which would involve Access or MySQL.
Some of the better CGI search scripts can do quite well just using their built in features though, so you may not need to go down the database road.
It all depends really on what it is you want the search to be able to return. Once you start to filter search results or drill into some serious detail you just can't beat a database underlying the site, buyt this could well be a sledgehammer and nut situation so let's not go down that road unless you think we need to.
Post again with details of what the web host supports and one of us will offer some posible links to downloads for you.