Never been a better time to get into this hobby really, as the range of products compared with say ten years ago is fantastic.
Before making recommendations, it's helpful to know a few details about what you are after.
1. What's your budget. Please don't say as little as possible- buy cheap, buy twice.
2. What size boat are you looking for. You've mentioned you want a GRP hull, so does that rule out injection moulded or vac-formed hulls?
3. What sort of subject are you interested in. Scale, semi scale, diesel or nuclear boat, what era, what scale would you like and what size would you like your boat to be.
Personally, for a first sub, I would recommend either a nuclear sub or a boat based on a submersible, and I would recommend you get a complete kit. Why?
Simple. These boats handle and perform well underwater, they tend to lack ultra fine detail, which may look nice, but causes drag when submerged and breaks easily. They also tend to be available in small handy sizes. Big boats might look impressive, but they're a handful in every way.
Also many of these boats are available as complete kits, and are often lower in price.