The first thing to remember is that SharePoint is just an ASP.NET application. It has a ton of framework, and various things like security, etc, built-in. But from your perspective, you can just build a Web Control, and basically deploy it right to the server. Check out this post from MikeG, and especially look at SmartPart on CodePlex. That will help you get up and running without too much fuss. Since you are using standard web controls, you have access to CodeBehind. The one caveat is that your assemblies have to either be deployed to the GAC or granted Full Trust in the config files - something SmartPart will help you through.
SharePoint Designer is just FrontPage, rebranded. It very much could help you with some basic stuff, and might be worth looking into if this is an occasional thing. But if you are going to be doing any kind of long term work, I highly recommend taking a class on developing SharePoint applications. It can be quite a beast, and you may find yourself in quicksand faster than you think.