I am planning to buy a mercedes benz c300 2014 model with around 80k miles on it, this is my first experience in buying a car, what are all the maintance issues and costs i should keep in mind before buying it

Mercedes dealer, other brand dealer, independent pre-owned car dealer, or individual?

Get the maintenance records. If they appear incomplete, be very wary as Mercedes Benzes require periodic maintenance to be reliable. Check the CarFax or other car history report by VIN for accident, ownership, and type of use. Enter the VIN into https://www.lastvin.com to see what equipment and options the car was delivered with.

It would be a good idea to take it to an independent Mercedes or German car shop and have a pre-purchase inspection that includes scanning for trouble codes.

Insist on a warranty if purchasing from a dealer. If from an individual, look into the cost of a 3rd party warranty before buying. Used Benzes can be quite expensive to repair unless you are qualified to do it yourself.

Now about the car (it is a W204)
Is it 4matic or RWD? The 4matic adds complexity and maintenance. The front drive shaft and the transfer case are potential trouble areas.
If the car is in or has a history of use in an area that uses chemicals to de-ice roads, inspect the rear subframe for corrosion. Some owners have had failures because of this. Mercedes is supposed to be in the process of extending warranty coverage for this problem.
Older versions of the W204 have had problems with the steering lock (ESL).
If it hasn’t had the spark plugs changed, it will need this around 100,000 miles.
It will probably require engine and transmission mounts around 100,000 miles.
A 2014 is the last version of the W204 and should generally be reliable. The M276 engine is used in many other models and is generally reliable, when well maintained.

Get in the car and drive it over a variety of road surfaces. There should be absolutely no rattles, knocks, or clunks. It should idle stopped in gear without any vibration. Drive it as fast as you feel comfortable on the highway. There should be no wind noise, very little road noise, and no vibrations. It should track straight and steer predictably.

Others may have more to add.

If this is the car you decide to buy, be aware that Mercedes dealers are expensive for routine maintenance. Independent Mercedes or German car shops are about ½ dealer price. I don’t suggest using the quick service depots on Benzes because they often don’t stock the approved fluids and filters, and they seldom know how to check the car’s internal maintenance system to see what other service is required at that point.

Your first car? Maybe you want a late model Honda.

I'm not trying to be snarky, but offering sound advice. These cars can be temperamental and cannot be neglected. While they are easy to work on, they are not cheap if you have to have all your work done by a shop.

I used a few VIN checkers including this one, https://www.lastvin.com . I always see REAR-VIEW CAMERA in the report. However, I bought my C300 new with no camera; I added it myself.

Be prepared to do the routine maintenance (oil change, brake fluid change, tranmission oil change, etc...) yourself if wanna save $$, otherwise, it'll be costly if you bring it to a Mercedes dealer because its parts & service are always high. Have it repaired at an independent shop if you are not comfortable doing it yourself.

Check out Xzero's thread for a maintenance guide.