Apologies in advance: this is going to be long!
I could relate to your dilemma, in terms of feelings if not in terms of exact same issues. I'm now 33 weeks pregnant and have no idea if we made the 'right' decision or not - I guess it was, since we made it....feels right at the moment, anyway.
I was 43 when I had dd, after years of trying, treatment, and miscarriages: she was my 'last ditch attempt for peace of mind'. The night before she was born, I suddenly realized I was going to agonize all over again...about whether to try for a second child. I was very tired the first eighteen months (no help till she was about 14 months old) and couldn't think about another till she turned two...at which point I was 45.
Thats when I agonized like you - I have no ongoing health issues, but did have high bp postpartum which scared me, and needed medicating. I look young, but don't always feel it - and I worried a lot about dd and whether I'd have the energy levels to meet her needs as well as a baby's. That still worries me a bit, but I have lots of help now - and dh plans to work part-time till dd2 is at pre-school. You say you can have lots of help too - I think thats key, really.
Before TTC this time, I saw a consultant OB Physician who knew about bp issues: he said go ahead. I saw my GP and asked for an ECG and 24 hour trace - she did both, and said 'fine, go ahead'. No one said 'don't'. I agree with those who say talk to your consultants and try to get a concensus on the impact another pregnancy/birth could have on your health...if they are saying 'go ahead', maybe its more your own fears you need to address. If they say 'hmm, I wouldn't recommend it' then you have to weight up some serious pros and cons.
FWIW, I put off childbearing for several years because of health issues in my thirties (liver disease) and opted for treatment first. It was probably the right thing to do, as I was too scared to focus on a baby - needed to deal with the health thing first. But...it meant I was 39 by the time I was 'allowed' to start to TTC, and then IF was a major, major issue. I wouldn't change things now - if I did, dd would not be dd! But it wasn't an easy decision, and I really feel for you.
As for the age gap, I wouldn't worry about that at all - my sister is six years older than me, and the only problems she had were more to do with how my parents dealt with (or didn't deal with) her natural, siblingy, jealous feelings than the fact that she had a little sister (and later, a brother). We are good mates now, and wouldn' be without each other for the world.
Being ill or exhausted all the time was my concern too - so far, so good: and contrary to my fears, my bp has been much much better this time around. Its impossible to predict everything! As for sleepless nights and being old - well, they say you need less sleep as you get older and anyway, that part is short-lived. And anyway, I don't think the age thing is the main issue - you are a mere babe: 40? Pah!
Just get yourself some reassurance on teh health front...it sounds like the most important bit..and clear info as to whether you need to deal with the kidney first, or whether it can wait. It does sort of sound as though it needs sorting out - if you were feeling better, and not ill a lot, you might feel a lot less anxious about TTC.
Big hugs and loads of luck with the decision.