NEVER BUY A MACHINE FROM EBAY.
Build it yourself, its not that hard, its basically lego these days.
The AMD APU chip's aren't that bad. They are better than normal on board, but you are still much better off with a dedicated card.
As someone who plays Train Simulator a lot, I can tell you that PC will struggle to run it if you want to turn up the gfx settings at all.
Some questions...
What resolution is your screen.
Can you wait til you have more cash available? If so, do. The best time to buy is when the next series of cpu/gpu comes out, they start selling of the previous series at ridiculously low prices. You want around £400 minimum to build a gaming machine.
Basic PC setup requires, CPU, GPU, RAM, HDD, PSU, Case, Motherboard.
After a certain point, the Motherboard, Case, HDD, PSU have little effect on gaming. The mobo does matter, but that involves a discussion about chipsets and goes way beyond the scope of this post.
RAM is less important than CPU, just make sure you get minimum 4GB, 8GB is preferable, anything over 8GB is great. CPU is less important than GPU, most CPU's are capable of throwing data round at silly rates anyway. For gaming, high cpu speed is preferable to multi cores. I'd rather have 2 cores at 3GHz than 4 cores at 2GHz. A lot of games can't make much use of multi-cpu's because, well its complicated, take my word for it.
GPU is the most important thing for gaming, get the most expensive you can afford, just make sure you don't exceed the limits of your PSU.
The last thing you need to know is that Train Simulator 2015 is one of the worst developed commercial games I have seen.
The simulation is broken for many locomotives as they add new stuff every year and dont fix the old layouts.
The graphics are poorly optimised. They said they were working on a new front end using Unreal Engine this year, and then promptly announced that they weren't.
I've seen this played on much higher spec systems than mine and it still struggles, despite not being that complicated a game.