"how on earth anyone ever knows if they are ready to have a baby" - er, I don't think anyone ever does really know, do they?
You admit you've never really had the opportunity before. BUT now you do. You have a 'wonderful relationship', you 'love children', your DP loves children (he has a son already so he knows what he's letting himself in for), he thinks you'd make a great mother. Honestly, it sounds like a good enough place to start to me! A fantastic place to start in fact.
All the other stuff sounds a bit peripheral. You didn't have a good relationship with your own mother - well at least you understand what to avoid and don't need to repeat her mistakes. You don't function well on no sleep - the 'night feed' stage really doesn't last long and anyway, MAT leave takes care of most if not all of it.
What do you mean when you say you think you're "too old"? Do you mean too old to conceive, or too old to be a parent to a young child? If you're talking about conception, you won't know until you try. Personally I don't think you're too old to be a parent, but then I wouldn't; I didn't have my DD (my first/only) until DP and I were 43. She's now 2 and is without doubt the most fantastic, amazing thing that's ever happened to me. I know I over-compensate (probably for fear some random person might think I'm 'too old') so for example, she always wears cool clothes (I doubt 2 year olds really need Diesel jeans etc) and we do loads of kids activities, so as not to be caught sitting around, like EVER. Yes I am sad she may well stay an only child (not for want of trying to rectify that situation) but I consider myself lucky that she's sociable and popular - tons of friends - so that's really just MY issue and not hers.
Short answer - I think you'll probably regret it if you don't.