Hello Mr Qaisar Azeemi
It seems that you have sufficient options from which to choose.
You can use the Colin55/unclejed613 ("simplest") design for which you need to determine a current limit circuit, ripple filtering (high levels of ripple current kill Lead Acid batteries), transformer design (will input transformer provide current lmiting?...gonna be big 'n' heavy) What is the battery application? Will temperature compensation be required? How would you provide that?
SCR-based design usually has large line frequency input transformer . Very noisy (electrical (sometimes physically noisy)), produces high levels of harmonic currents and RFI/EMI. These must be addressed as well as output filtering; similar issues to the above. Current limit and voltage regulation and temperature compensation can be built into control circuitry. Generally design is complex result is big, heavy and expensive. What is the application of the batteries? Moderate efficiency.
Switching regulator can provide all of the bells and whistles but is complex in design and operation. It is however the only way to get high efficiency in these ac to dc converters. The outcome is a relatively small unit, which is suitable for paralleling, so smaller, cheaper modules can be added to give higher power/current outputs.
I would add only that it is essential to begin a design journal ((diary(if you don't already do so)). Describe the design problem firstly...what are you looking to produce:
48v30A battery charger for 48v300Ah battery banks...what type of LA batteries flooded, VRLA, GEL?
...are they a stationary, standby installation...are you float charging them?
...is there a standing load connected to them?
...if not a stationary installation, are they traction (forklift, golf cart) batteries?
...do special standards apply to the installation...telecommunications...power supply authority?
READ ALL YOU CAN ABOUT LEAD ACID BATTERIES AND THEIR CHARGING REQUIREMENTS...YOU CHARGER HAS TO ACHIEVE THESE REQUIREMENTS.
So far your design journal should have a description of your design brief; absolutely all you know about LA batteries and their charging regimes.
You should go on at this stage to begin evaluating the 'design difficulties' for you. Get help for these or read up on them. Continually update your design in schematics and notes so you always have something to show your boss. In the early stages develop parallel designs. Always make notes on why you decided one way or another, and progressively develop a costing schedule on designs to ultimately justify your design decisions.
You are considering charging 48v300Ah battery banks here. They are expensive and must be kept fully charged and maintained in good condition; they are not batteries for toys or torches.
Your task is an involved one and would usually have a number of people involved. But by your postings you are alone in this. I couldn't estimate how long this will take you; it depends on so many things. If you keep a journal to instantly support your decisions, assumptions, costings, your boss will be able to see your efforts and progress.
As regards a timeline, I don't think you can do so for the whole project, but you should attempt to define how far you will have progressed in the first week/month, then continually update and expand that as you progress.