All excellent advise above.
The bonus of a home brew chapter is that you get to decide which chapter tactics to follow, and can change your mind if you don't like them!
Just in case you haven't come across that part in the codex yet, chapter tactics are a set of special rules unique to each of the founding loyalist chapters that don't have their own codex. (Blood Angels, Dark Angels and Space Wolves have their own codices. Black Templars used to but now they're back in the big group hug). They're mostly straight forward and affect most of your army.
I made my army before these came in, and I'm still adjusting parts of my play style to suit (I play using the Raven Guard chapter tactics). I don't think anyone is likely to stop you playing as a different chapter every now and then, and it's indeed fun to try something different every once in a while. (Playing your Ultramarines Army as Salamanders for example, just make sure you're opponent is fully aware before you melt his face)
I guess the easiest advice is to think of your preferred playstyle first, then pick a set of chapter tactics you think will support it and give it a whirl.
It's also worth noting that chapter tactics also limit your choice of special characters, although you can work around this using allied rules. Just don't go buying all the special snowflakes in the book as your starter army.