So I have a lot of infanty. A Lot. I have been collecting IG since the days of 2nd Edition. Many of them are the original box set models you got (I loved those boxes. Each one with a different 'regiment', sergeant, special weapon and heavy weapon. I seem to recall they were decent value for money too).
Some of them are the plastic sets. Both the original Catachan plastic set (which I think receives too much poor press. It was and is a good kit. Yes the sculpts make them all look a bit too Rambo ish and their armour seems to be mainly big muscles, but hey, that is what the Catachans were all about!).
You may be able to tell from the painting styles that some were done way back, 20 odd years ago. Painting in big blobs, straight from the bottle, with little or no consideration for the details on the models. The better painted ones are the ones done in the last few years, when I had more inclination to do it as a hobby of painting rather than getting them on the tabletop as fast as possible.
Most of them are GW models. There are the occasional non-GW ones such as the casualties and a couple of the command models (can you spot which ones?).
Some of the models are ancient. There is one I use as an assassin that goes back a fair way, before 2nd Ed. The original stormtrooper plastic models I just love and I wish I had picked up more of the metal versions back then. The Frateris Militia models make an excellent basic guard model in my opinion and can be swapped out for militia, PDF, conscripts, Penal Legion or Necromunda.
I believe I have a couple more Ogryns lurking in one of my tank boxes and I must get out all the heavy weapons one day and match them up with the loaders.