I have been building PC's for 10 years now and I can honestly say you are not going to get a brand new PC with great specs for that price.
Personally if I had that budget, I would be looking on eBay/Gumtree for a 2nd hand PC as you will find some pretty good deals considering the market for selling used Gaming PC's isn't that great and a lot of experienced builders will take it all apart again and sell the parts individually to get maximum return.
It really depends on what other games you are wanting to play on it as OMSI isn't a demanding game that requires a top of the range PC. It will stutter from time to time on any pc just as the game isn't very well optimised.
My current PC is a SCAN 3XS system which cost me over £2500 and depending on the map it sometimes has a moment.
What I will say is, do not buy one of they prebuilt systems that have all the fancy LED lights in a spaceship looking case for £200/£300 as they are mostly full of unbranded crap that will not last long.
With your budget, I would expect you to be able to source a 1050ti/1060 with decent ram and cpu.
Good luck.