What I wish I knew before hand? It can be quite a lengthy process. Almost a year from my initial enquiry till all training and pre inspection visit was completed.
The cost - this will depend on your LA but when I did my training (2011) the cost was £40 for the training plus a fee to the GP for my medical, setting up initial paperwork, extra resources e.g fire blanket, first aid kits plus high chair, stair gates etc if needed. You will also need to register with Ofsted (£35 IIRC) and you will need PLI. Plus NCMA membership if you choose. Yes you can claim these as expenses but you still have to pay the money up front. A friend of mine is looking into doing it now (different LA) and has been told the cost will be £10 for the initial introductory session then £250 if she decides to go ahead and complete the training.
What I love is being able to be home with DD who is 2 and take DS, 6 to and from school, not struggle for childcare in the holidays and just generally be around.
What I find hard - not much really, personally I feel much less stressed compared to when I was working although I only mind 3 days a week so still get a couple of days with just DD. It can be quite physical though, especially running around after babies and toddlers all day - I do sometimes feel like all I do is wipe noses and change bums and play (and tidy up - lots)
Some find the paperwork a slog. It can be hard to get your head around at first but once you find a system that works for you it doesn't have to be all consuming. I use an online system for all my recording of obs, pictures etc that the parents can also register with and see exactly what you upload.
You may find your house gets taken over but again there are ways around this. Good storage for your toys etc that can be put away at the end of the day.
It is also worth remembering that you can go through all the training etc. and then be sat there waiting for someone to want a place. It was 6 months before I had my first mindee in place. Your FIS may be able to tell you what demand is like in your area. Luckily for me this is a short term career until DD starts school and I finish studying as my earnings are much much less than when I was WOH, however many can and do earn much more.
Sorry, that was a bit of a mammoth post but hope it was helpful!