[DiveDevil1966]

Here we go.
You have a lot of questions, which are very good, but then at the same time there are some concerns. Before I am answering your questions I would like to say, that I would not focus on CHEAP for what you want to do, as QUALITY is so much more important. There is many places in the world where you can do what you want to achieve. Look for value for money and in particular for good quality training. I am in the industry for a long time and worked at all levels and I have seen so many instructors which are just not worth the plastic their credential are written on.
But let’s start:
It is correct that you must be a qualified diver for at least six months before you can attend a PADI Instructor Course. This means that you should be this done NOW, or at least as soon as possible. From the day you are certified the six months start to count down.
The 50 or so dives you have already completed are good experience and there is no problem to have them logged, but will not credit to your diving courses, but can easily be logged and show the amount of dives you have completed overall.
There is many training associations and it depends a bit of your demographic area on which one you want to go with. For example SSI is far bigger than PADI in Italy. However PADI is the world largest training agency and if you intend to work as a diving instructor anywhere in the world then I suggest going the PADI route. I started with another training agency and climbed the ladder with them, and then just came to the point where I had to do the cross over to PADI to ensure flexibility on destinations and job's available.
It is easily possible to cross over from one training agency to the other to safe some time, but I am not sure whether this is beneficial. It does not change anything as far as the six months are concerned. As soon as you start to get to the Dive Master and instructor level, I think it is important to understand the system you are training with. This will make you a well rounded professional, and will also make it much easier at these levels. I think you should start with one agency and go all the way. Do some research in your area and which agency is big there and then contact a few people and see what they have on offer.
Let me know in which area you life and I might be able to get you in touch with a good diving centre or I might even be able to explain to you what we are doing in cases like yours :-)
Please let me know if I can be of any further assistance, and I am happy to answer any questions you may have.
Remember QUALITY OF TRAINING is more important than CHEAP.
Always safe diving
DiveDevil