We started ABA at 2.10 (DS did not regress until over 2 and dx at 2.6). I can't answer your questions about how it would work with a much younger child, but would disagree it is just behavioural. DS had a lot of language which he stopped using and his language was the first thing to come back greatly assisted by ABA. PECS is ABA based approach. When his language improved he also became more social, which I wasn't expecting. He showed more interest in staff at nursery and interacted with them more. His behaviour has not really changed with ABA - he was passive and easy going before and still is - we work on behaviour during ABA ie because he has to sit, listen, pay attention etc which he does not like; but he does not have any behavioural issues (aggression, problems with transition, meltdowns) etc but ABA has still been really good for language, social skills, attention, play skills - we work on all of these. I would think you would have to talk to actual ABA providers about how it would work with a young toddler, because it would be quite different to what we are doing. Some have waiting lists. (Most things with autism have a waiting list). Also go and watch a session.
It might be worth contacting Treehouse (ABA school in North London) they take children from age 3 but also have charity / info / advice etc
Also if looking at PECS look at Pyramid website. If you don't want to sit on a waiting list you can go on a PECS course yourself or even take your child to Brighton and they will teach PECS to you / child intensively. Our local speech therapy team refused to teach PECS to a friends child until he was 2.5 because they said (wrongly) he would not be able to do it before then, but I know Pyramid staff have worked with much younger children.
I would not be able to do ABA and work full-time. Our provider insists we are present in the house when tutors come. So you might have to get a tutor to go to a childminder / nursery etc. However what you would miss out is seeing the tutor work with your child and learning those skills. Really the parent has to know how to deliver the programme so you can use it outside tutor time eg when at shops etc using the same skills to promote language etc. Some tutors will do weekends, but supervision etc tends to be during the week, but could be evenings. Ideally both parents would need to be at supervision sessions. It is a big commitment.
We do most therapy hours ourselves eg we are the main tutors so we chose to work less and do more ABA ourselves; otherwise I would be working but paying someone else to do ABA. But would have to pay twice - once to pay a childminder / nursery and once to pay ABA so I wouldn't be better off and I wouldn't be learning how to be more effective with DS.
Sometimes you can get volunteers eg in London you might be able to get psychology students to be tutors as volunteers and some people use family members / friends. There are ways of cutting costs.
What I would also say is that my son does not tolerate long sessions at nursery. He goes 3 mornings and that is enough for him. His behaviour tends to deteriorate and he gets overloaded. This time last year I was working part-time, looking to work more / earn more etc and then DS regressed. I have to say financially it has been disastrous. We get DLA and Carers allowance but it does not make up for a career. I have turned down 3 really good jobs in the past month because I would not commit to the necessary hours they wanted me for. Even when DS is in school, I won't be able to work fulltime as he could not cope with before / after school clubs etc and nor would they necessarily have the skills to cope with him. I have resigned myself to not having a career again for the foreseeable future. i do some freelance work but DS is really a fulltime job. Not just the actual therapy but all the stuff that goes with it - handover at nursery can take 20 minutes not 2; we have reams of paperwork re appointments; paperwork about his education needs and a battle with our local authority over his provision. That probably sounds really depressing but its the reality of life this past year. I spent several hours on Boxing Day working on replies to the local authority proposed statement. We have earnt far less and paid out far more this year (autism is an expensive business). The other thing I would say is that its hard on you, the worry and anxiety and coming to terms with it and I have been relieved actually that my work demands were manageable. I think starting a new onerous job whilst having this over you would be a tough one. My parents help us out, they feel useless being far away so give us a little bit of money each month towards his ABA. So talk to those around you they might want to help.
My sister is in USA and her son had speech delay (not ASD) and they both worked full-time and so organised play therapy and speech therapy through the nursery. Her son did improve but his problems were mild compared to DS and I think he would have improved more quickly if his parents had been more involved in the therapy and knew what they should be pushing at home, as delegating it entirely to others in my opinion really reduced the effectiveness. You could video some sessions and watch them back etc, but it wouldn't be the same as being there, I find the supervision essential. Also its a chance for us to say "DS has started doing this this week" and then they can build on that or address it and you have that direct dialogue with the specialist. I think the research shows parental involvement is a key factor.
I would recommend "A work in Progress" by Ron Leaf to read as it explains what an ABA curriculum includes and you will see its far more than just dealing with behavioural problems.
Another thing to consider is specialist nurseries or you might find a spec ed teacher who is looking for part-time work eg if they have their own children. I know one childminder who used to work in a SN school.
But I would agree, when they improve, being skint is worthwhile.