Oh lordy me where to start!! I have a 2 1/4 year old DS and am on my 4th change bag..... As long as there are plenty of compartments then you should be fine. Know where you're DH is coming from though, so perhaps 2 would be sensible at first.
First one was one from John Lewis, which was fine until it fell apart - but in hindsight didn't have enough space/compartments anyway! By the time it died we were still at the bottle stage (gives you an indication of its longevity!) and we were on disposables so bag and binning the dirty ones where possible. Always had the bottles in a separate compartment so they were separate.
Then moved on the the avent backpack, which was fantastic up until my DS didn't need 6 changes of clothes, bottles, beakers, dummies, baby food jars etc when it got a bit big for a 2 hour outing - this is a great first bag as the bottom section zips open and has a velcro-ed in change mat, with space for clean and dirty nappies, wipes, bags, creams etc etc! And loads of space inside for bits and pieces, plus an insulated bottle holder and a front section that holds 4 small jars of baby food - I have to say if I could only buy 1 bag and especially with using washables would buy this one.
So moved on to an adapted vanity case of my own for compactness (is that a word? whatever), similar to the baby bjorn clamshells (but not as nice!) which was great and as it had 2 sides then the clean and dirty stuff was separate.
Then swapped to washables, and boy do you need more space! Was considering moving back to the avent one, then found a baby bjorn for £5 at an NCT sale. This is great, but they are costly and TBH not sure if there would be enough space for a newborn on washables. At least I only have to take a couple of washables with me most of the time now! If going out for a whole day then I tend to take his nursery dirty bag with me (tots bots dirty bag) as it holds the dirty washables and keeps them separate.
Would suggest that you get a couple of terrys (of folded up t shirts) and take them with you as this will give you an idea of capacity - dispoables are really slimline and most of the bags are designed with them in mind rather than washables.
Incidentally, if you are looking for washables and are in the East Surrey area let me know as I know the Lollipop adviser well and can put you in contact!
Good luck!