Sounds like a plan. I should however point out that there are specific pumps for specific jobs, so for a pond filter you need a filter pump, filter pumps are designed to handle "solids" and pump them to the filter, and it is the job of the filter to catch and hold solids. Filter pumps send high volumes of water at low pressure to the filter, where as a fountain pump is designed to pump "clear water" (No solids) at great speed in order to operate the fountain nozzle. If you use the wrong pump for the wrong task, it often will not work, I have seen people ask why a fountain nozzle does not work as the picture on the box shows, its because they have a filter pump, which is just not made for the task, I have also seen the opposite where people complain the pump blocks every day, its because they are using a fountain pump to supply a filter, fountain pumps can not handle solids, and soon block.
It really depends on how inventive you are. I would suggest you draw what you are thinking of on paper then see what you can find that fits what you have drawn. I should point out: A bottom drain is NOT for catching leaves, as leaves often float. For leaves you want a skimmer. If you are having a bottom drain (good idea) then where ever it runs to should have its top slightly higher than the highest point of your pool, if you don't and you have a power cut or the pump fails all the water will come out over the top of what the bottom drain is connected to as it is lower than the pool.
Green water is caused by sunlight and too many nutrients causing algae to grow and multiply.
The best way to remove it is via a UV and filter. Filters can only filter so much then they must be cleaned, you should refer to the instructions for your filter, but generally with pressure filters the sponges will be clean when the water from the "waste pipe" runs clear.
You should change the UV tube at the beginning of the season, since they are on 24/7 average life is 6 months. (When did you change yours?)
You should NEVER turn the pump off even through winter.
It would also be a good idea to move the pump as far away from the returning water as possible
The idea of a filter is to give bacteria somewhere to live and multiply as well as remove large particles from the pond. To do this they need a good media to live on and a constant source of food.
The food is pumped in by a filter pond pump, filter pumps can handle large solids which the filter will remove. If the pump flow rate is too fast the filter will not work properly. I have yet to hear of a filter that is fed too slow. (But I suppose it could happen)
So to answer your question, you need a filter pump that does not exceed the maximum flow rate of your filter, and one that will pump the entire pond volume through the filter once every 4-6 hours.
I do not suggest a specific pump as I am still very biased.
This is one of those questions you can ask 10 people and you will get 10 different answers from. All you have to do now is find 9 more people and ask them.
My take on it is get a big stick and poke it in it and twirl it round like candyfloss to get it out. (I am being serious) Some will say it's because of the nutrients, some will say it's a "bug" brought in by visiting animals/birds etc some will say have lilies to give more surface cover, I could go on. Me? I really have no idea why it occurs, it just does. I do know that some ponds will never have it for years, then poof, loads of it. I do know most chemical cures are just a money spinner for the company who make it. (Hence use a stick and treat it like candyfloss) I have also yet to see a 100% guaranteed cure, and that includes barley straw.
It would also be nice if you were to stay, but I doubt you will, please prove me wrong.