I've got a great idea and a rough idea of what I want, but the tutorials I read all assume I have a game engine and have messed around with it a bit. Honestly I have no idea what engine I even need

Since I have a lot of ideas I want in the game I think I will need to keep the core system simple to compensate. After thinking about it for a while I decided on a top down at-an-angle 2D game. An example would be Project Zomboid:

http://small-games.info/s/f/p/Project_Zomboid_1.jpg

Actually it was inspired by Project Zomboid so I expect the game will look very much like that screenshot so please check it. It's basically like The Sims with buildings and characters and furniture etc but the point of view is fixed and the camera cannot be rotated (is that 2D?) so I'm guessing that would be a lot easier to program than something fully 3D like The Sims

Here's a short gameplay video of PZ, skip to 0:18 for the gameplay:

I won't be creating a zombie game it will be medieval but I want it to look and play something like that

PZ uses an in-house engine. I don't start uni til September and I expect creating my own engine won't be one of the first things we jump into. Is there something I can use to get this kind of effect?

As you can imagine, this question is asked at least once a day. I would suggest you look through these search results first and see if they give you the information you're looking for:

what game engine

To be honest, 2d isometric is kind of a pain to work with. Depending on what you're doing, you may want to think about just doing a 3d game with an isometric camera.

I would try out Godot engine for a game like this. It looks very promising.

http://www.fifengine.net/

FIFE (the Flexible Isometric Free Engine) is designed around isometric games, mainly, unlike other more general-purpose options. This means it will help deal specifically with a lot of annoyance you might otherwise have building the "isometric" aspects of a game in a more general 2D or 3D engine. It comes with editing tools, like a map editor, saving you from having to build or repurpose one.

Originally it was designed to emulate the early Fallout games (one of the F's used to stand for Fallout). I see it now supports top-down maps, which is new last time I checked on it; one of the developers used to be a regular in #gamedev.

It's a very mature engine, having been around much longer than Unity and the like, and is still actively maintained.