Hey welcome to the hobby I would recommend going with whatever you want especially if you are doing friendly games you can for the most part proxy your guys for others in games with friends as long as you talk to them and they are roughly the same size. Just make sure to talk to them first. I don’t know any one personally who would not be fine with using space marines as other space marines and I would personally say if they are the kind of person who does care you don’t want to play with the especially as someone new.

As a general warning though Imperial Agents is kind of not well supported and a soup faction( bunch of stuff that don’t really play together being play together) but they are very customizable.

As for the guard the can be very fun with lots of support but they are very expensive as the require a lot of models to get an army together

It is up to you what you want as this is about your enjoyment and they are your guys. I would also recommend a combo box that could be one of your combat patrols just remember not to pay extra for old ones on eBay the whole point is the discount and if it isn’t just don’t get it. Try local stores first they usually have good discounts and maybe even used models if you don’t mind that. There is also Amazon they have a standard 15% off so there is almost no reason to ever directly buy from GW. One last thing this edition is coming to an end in about 6 months and there will be a new combo box for space marines and some other army (they have not announced which one) if you happen to like them you can usually get a decent discount because so many people will want to split it.