If you don´t have that much money, I can give you and advice, but I don´t know if you are gonna like it. First of all, search for pics of the Death Korps of Krieg soldiers, but not just look at them, pic a paper and a pencil and start drawing rough designs of what they wear. They carry a lot of stuff, so I would say a good idea is focus in some of the things. You will end realizing that you need, at least:
The face mask
The Helmet
A trench coat
The belt and straps
A lasgun
Some pouches and maybe a grenade
Cloth bands, to use them as gaiters. Yeah, they do not use cavalry boots, as armageddon steel legion. Better for you.
I recommend you to ignore the showel, bayonet, backpack etc. It will be too much work for your first costume and you will end up getting bored an putting low effort in the final steps.
Once you have a general idea of what you need, the fun beggins. This depends on how much confidence or hability you have, but I will tell you something I have realized about imperial guard costumes:
-Many people choose them because it seems to be easy (or easiest than, you know, a terminator armor) and they end up puting low effort.
-People use as much military surplus as they can, because is cheap and you don´t need to work on it, just wear it. There is no problem if you use some pouches or a showel, few people really look at those things, but in items that are the focus of the cosplay, like the mask or the helmet, the things everybody can recognise, better not use a soviet GP-5 or any other shitty gasmask. They just does not look accurate. People who use too much military surplus ends up looking as a random soldier of an early 20th century conflict but with an imperial aquila sewed to the coat, for some reason.
Said that, if you find something wich is objective very accurate, use it. For example, the trench coat. You can use a military surplus soviet or bulgarian trench coat, of course. If you are just starting, you are not going to sew the entire uniform from a bunch of fabric. But sure you can modify it. Most military trench coat come in notch lapel or mao neck, and are double or single breateasted. Try to pick a double breasted, 3/4 long, notch lapel coat. Then use an ironing or even a bit of steel wire to form the neck as it looks in the miniatures. Sew the front bottom corners up, forming a triangle, as the wwi french soldiers also did. Replace the buttons if they are not very accurate. If you feel confident enough, even decolour the whole coat with bleach and then use blue dye for clothes to make it look like the forgewold minis. If you do the same with the trousers and the cloth bands you are gonna use as gaiters, so everything is the same color, congratulations, you did it better than the 90% of death korps of krieg cosplayers. The helmet and the rifle are tricky. I´ve seen some very cool and accurate replica of the helmet at etsy for like 60 bucks. Is what a stahlhem replica costs, but is more accurate. They have the mask too. There are also 3D printing files available. I like to do all my replicas the old way, with a ruler and a lot of paper, and cardboard prototypes to test measures, and you should try it. You may end up with something cool.
The respirator looks also a bit difficult, but in fact, is a leather box vith some dials. I wold recomend carboard covered with faux blown leather, a bit of silver paint and a wash with black ink.
Aaaaand I think that´s all. Just remember: try to do or find an exact replica and you will create a very accurate gear. Try to do something that "does not look so bad" and you will do a bad cosplay.
WH40K Pepakura File Library 
