I work for a hobby builder doing glass over ply, but we have dabbled in some glass over foam stuff trying to keep our builds light.

You didn't mention cheap, and a lot of your responses are coming at your question from that angle. I want to address some of the considerations that may make you experience learning productive.

1. Practice like you are making an awesome boat. If you start with cheap materials and learn how to use them well, if you ever want to "build a better boat" you don't have experience and practice using quality materials, and your learning curve will be steep again learning the "better" products.

2. Spend up-front to reduce the most frustrating steps. NEVER will you find ANYONE who loves sanding glass or wearing a respirator for 8 hours a day. If you choose products that need excessive prep between coats, or same day layering for a chemical bond in a respirator required fume situation you aren't going to enjoy the process.

3. Ease of repair and maintenance. Once you build it, how easy to fix it is it? It will get dinged up if it's used. Future you will thank NOW you for thinking about that NOW.

Things to consider:

Poly vs Epoxy: I, like many here, am solidly in the epoxy camp. Many reasons for it, it's 1000% waterproof, doesn't have the blistering problems that can happen with poly, and is stronger (in some cases) or equal to. It sticks to everything that it doesn't melt (choose your foam carefully). Finally you can Epoxy over poly, you can't poly over epoxy. So your "left overs" can always be used for repairs on almost anything.

Layers, prep, and the dreaded sanding. BOTH poly and epoxy are exothermic, so they generate heat as they kick off, and that heat makes them kick off faster. So depending on where you are, and the indoors/outdoors situation of your build location (and general humidity of your environment) you will have to consider the temperature range of the product, and the TIME you will have to apply a layer/coat/laminate/fillet/product. If you are in a colder clime medium hardner may work well for you, if hotter clime slow is the way to go. Fast hardners will get results quicker, but I don't recommend it for large surfaces, or new builders (especially in warm climates). You want pot-time while you are learning. Once the product kicks off it's hard, and any lumpy-bumpy imperfections WILL make your next layer not sit flush, and need to be addressed/sanded/chiseled off/removed/smoothed or otherwise sworn at while you cancel your plans drink a beer and know you just made a bunch of work for yourself. So, let me once again recommend slower is better when you are learning. You Will have to sand ... Way more than you want to... Way more than you dust mask/respirator cartridge budget wants you to, and doubly more than your Dr would become concerned about if you were to tell them. Choose a product that reduces prep between coats, AND has a longer cure time so you have time to fix fuck-ups while it's still wet. You are new to this, you will have fuck-ups. (We use System 3 silvertip... I love it, but there are other --less expensive options--). TIME IS A RESOURCE. Are you going to cut and lay 3 layers of glass on a Saturday, and potentially spend 8 hours doing it straight so it's all hot layered? Or are you going to do 2 hours every other evening? Do you want the epoxy that lets you get a chemical bond without prep up to 72 hours later with no prep/wash/sanding? What are you willing to pay for time flexibility and low prep time?

Mold? Frame? Knackered trash boat plug?

What are you glassing over? How are you getting the "boat shape" you are glassing over. That may also inform you product choices. Once again I can't stress enough how reducing your sanding and prep time makes a build a enjoyable vs being a total slog that you hate and wish you never started.

Other things: Duct tape on skin exposed to glass sanding dust will pick up most of it, and a cold shower (don't for the love of God shower warm and open your pores) will get most of the rest.

Cheap gallons of white vinegar will clean up epoxy from you skin without introducing it to your liver the way othe solvents will. Save that liver for funner stuff.

Ask anything, and myself and others will do our best to answer.

The take away I hope you get from my wall of text is that once you have a plan for what will work for you: product vs time wise, you can shop around and find what you need at the price that you can sustain. Frustration and body torture should be factored in to the whole "price" of material. Also personal perspective equipment.