Hi Jai
I ran Märklin analogue trains from 1961 until 1986.
Then I ran Märklin digital using 6020, 6021 Central Units and then 60212, 60215 and now 60226 (CS3)
Since 2013, I went one step further and use Rocrail (computer-based software to drive trains and manage occupancies

If you have a mojority of Märklin devices, I would recommend a CS3 as it can:
- drive digital locos under the Motorola, DCC and MFX protocols
- parameterize in a very sophisticated way complex parameters all with user friendlyness
- program sounds on Märklin decoders
- access the entire family of digital devices.
- benefit from permanent Märklin improvements
- available power is up to 100 W for a CS3 and / or for each additional booster
- automatization of the layout using the S88 modules
- of course 120 VAC and US plugs are not an issue for Märklin

In the Märklin range, you have a less-costly approach in using one MS2, with its track box and its 36 W powersupply

Could you run more than 2 trains at a time.?
- with a CS3, this is easy as you have already 2 controllers (2 different trains)
- with a MS2 it is trickyer as you need to select another train "on the fly"
- with a CS3 and automation ("Events") and using signals and their stop zone, you may drive a large number of trains at the same time
- using some tricks, you may even manage your CS3 to send digital orders to your trains (start, stop, set speed, set horn, etc).

How to convert from analog to digital?
- usually locos are converted first. In the Märklin world, this is about replacing the direction relay by a mLD3 or mSD3 decoder.
- the loco conversion may include the motor conversion (from an AC motor to a DC-5-pole motors
- the solenoid-operated item may remain analog at first and then converted to digital using M83 decoders (60832)
- if you plan for mix operation on your layout (analog AND digital) be sure to separate the 2 zones and even remove connecting track to avoid a digital disaster

How much power do I need?
- Power is mostly absorbed by motors and light bulbs so old locos need a 10 VA power while recent, sound ones need 2 to 3 VA
- try to convert most car lighting to LEDs as the needed power is much less
- try to avoid using very old Märklin locos: I have a DL800, CCS800 with a very large motor that needs 20 to 30 VA

Here is my layout powered by a single Central Station (CS2 at the time, now a CS3) with a 100 W power supply for 26 locos and plenty of lighted trains.
The total need is between 2.9 to 3.3A when a total of 5 A is available

The automatic operation you see is managed by Rocrail (one step further beyond digital). Let me know if you are interested
Cheers
Jean

Since you are beginner of the digital i recommended you start with a MS2, if not less you can also use other competitors digital system.
Märklin allows DCC protocol in the new digital locomotives so that´s way it´s good to have a system that support DCC too.
You can do any thing with the digital system when you control the layout and there is a lot of fun.
Just take a small step and if you have friends that also plays with the train toys, just ask them about digital.

Marklin book on running digital will give you most you need and is worth buying.