As a note, I am not against 10+ gallon systems, but going from a first brew to 10G all grain requires buying or making a ton of equipment and it would really help to also understand yeast pitching rates, how much grain will need to be purchased, storage space, cooling temps since you are now doing a full boil, mash steps etc. Really just basic all grain stuff. None of this is impossible but making a mistake with 5G is less problematic than 10G and also a lot less expensive. Getting a stuck mash has a higher probability of happening in the first few batches, especially if you crush your own grain. Having a stuck mash with 5G is much easier to fix or toss than 10G. The first and only time I had a stuck mash was the first time I crushed my own grain and didn't condition. I crushed too tight but I wanted that extra efficiency and couldn't recover from it. I was only making 5G and I tossed it swearing up and down. It was an extra variable which I was tuning and I failed the first time and haven't since. Wasting 20 to 30$ in grain is more expensive than 10-15.
This is not impossible but I don't want to see someone work their asses off for a few weeks and spending hundred of dollars preparing everything that they think they need and then on brew day realize that they needed this extra thing that they didn't think of. This is why reading books helps and going to 5G, adding a bit more and going to 10G is how I would do it. Almost all of the equipment is useful for both systems and if you already have a few keggies, why not get the process of doing good 5G batches and then expanding to 10G once the basic process is worked out which REALLY is NOT HARD.
That being said, you need to worry about yeast pitching rates or buy a lot more yeast. I can not (and have yet to) stress this enough. Getting enough yeast for a 5G batch is hard from a smack pack, getting it for a 10G system is harder unless you pitch from slurry or top crop which is a great idea to try. Basically, this process does take some time to learn as a hobby and jumping right in to making 10G of imperial porter with vanilla beans and whiskey soaked french oak chips is going to suck if you under pitch and it gets infected, or you forgot to sanitize something, or any host of other problems.
I am all for people expanding their setups and systems and making the best damn beer that they can in a quantity that they see fit, but saying that this is going to be cheap and easy is an outright lie, even with the 3 kettles already owned. You can always go out and buy or make all of the equipment that you need, but in my experience figuring out what you need and when is something that you learn through doing or reading books, or talking with people at the LHBS.
This might sound very pessimistic and difficult but I really don't mean it this way. You don't start doing complex mathematics without knowing algebra. That is basically my point. Go nuts as much as you want, but be knowledgeable as to what you need to do first. My first few all grains ware terrible. My dozens since then have been great. Really this is only to say, start small and learn as much as you can and add things slowly. This way when something goes wrong you know exactly where it went wrong and know how to fix it.
