{help} Is this laptop good "Asus K55VM-SX086V Laptop"

It depends on what your looking for , max budget , needs and expectations.

If your looking for a gaming laptop , then this ones a considerable buy.
If your just looking for something fast for general usage like web surfing , work , etc , get something cheaper and add a 120/128/240/250/256GB SSD like the Samsung 840 , Plextor M5S ,etc. The speed of that laptop + SSD will be far more than an i7 laptop with HDD. So yeah , get anything you want with a SSD.
You can get it installed from a local engineer with ease.

Also I would recommend you building a computer yourself if your gaming.
DIY PC's are always fast for gaming and affordable in price/performance ratio.

Lets answer questions in order -
Yes , You can replace the HDD with a SSD. Its simple. Although since I haven't opened the laptop , cant say. Your better off going to Youtube for some proper help.
In most cases , laptops have a removable back panel , and then everything open. Some have special compartments. Your best bet is a local engineer , as it will go safer and unless your confident on yourself.
Yes , ASUS is a trustworthy brand. They have some great products and reliable service out here in India. Haven't had a experience myself , friends said this.
SSD have no moving parts like traditional HDDs. They consist of flash storage on NAND , which is faster compared to that of traditional HDDs , storing on platter(s). They have higher life than HDDs in average cases , hence more reliability.

If gaming is needed , I would prefer a DIY PC with the Phenom II 565 BE + HD 7790/R7 260 + 128GB SSD , if the laptop will just be at home. Else drop gaming to lower priority , or get a slower basic performance or burning a hole on the pocket.