Mode of transport to Hotel Monterey Grasmere

Which is the best way to get from Kansai International Airport to Hotel Monterey Grasmere Osaka in Namba?

Naikai Limited Express or OCAT Bus 11?

If we take the Naikai Limited Express to Namba, do we need to walk far to reach our hotel?

Understand there is also the OCAT Bus 11 to get to Namba and is nearer to JR Namba and nearer to the hotel. Is this a better option? How much and how to purchase the ticket and how long does the travelling takes?

The bus is more convenient because it is closer to the hotel. The walk from the train station is longer and not straight-forward:
http://www.hotelmonterey.co.jp/en/htl/grasmere_osaka/directions.html

I personally would probably still take the train, though, because I prefer train travel over bus travel.

The bus timetable and fares are explained here. You buy the ticket at the airport:
http://www.kate.co.jp/pc/e_time_table/e_ocat_tt.html

The bus is a much better option. No need to handle your luggage and no long walk with luggage through a confusing station. Do yourself a favor and take the bus.

Which is the best way to get from Kansai International Airport to Hotel Monterey Grasmere Osaka in Namba?

That depends on your preference. Some prefer bus. Some prefer rail.

When on the day will you come?
If your plane is scheduled to arrive earlier than 20:30, both limousine bus by KATE and rail by JR-West sound good.

On the current timetable, the last limousine bus for Namba OCAT departs at 22:25 from No. 11 bus stop.
The one-way fare is 1000 yen. The roundtrip fare is 1800 yen.
The route requires about 50 minutes, though it depends on traffic conditions.
You cannot reserve your seat in this limousine.
- http://www.kate.co.jp/pc/e_time_table/e_ocat.html

On the current timetable, the last Ltd. Express "Haruka" departs at 22:16.
It's worth checking whether you can get an ICOCA & HARUKA package. With a ticket in this package, you can get from Kansai-airport to JR-Namba at 1000 yen.
The route requires about 1 hour or less.
You can take a "Haruka" train to Tennoji with this ticket, though without reservation.
- http://www.japan-guide.com/e/e2359_icoca_haruka.html
You can make a transfer at the same platform in Tennoji from a "Haruka" train to a Local / Rapid train for JR-Namba. The section Tennoji -- JR-Namba requires less than 10 minutes.

I will arrive at Kansai airport on sunday morning at 8.50am