Mass gainer gives u extra calories and carbohydrate, which u can simply get it by eatting brown rice, pasta, sweet potato and etc (all the complex carbs) which is not that expensive. However whey only have Whey, minimal carbs and some bcaa.

If u would have ask me, i would say go for whey, and mix it with glucose (or any simple carbs, higher the gi the better) for ur post workout and the rest of the day, just consume more protein (remember u need 1g of protein to per lbs of ur body weight). Add on healthy fat (coconut oil, oilive oil, sunflower seed, pumpkin seed and etc) on ur daily diet for extra calories for u to gain weight.

Remember, those supplement is still supplement, real food is way better than those powder

The result will be the same with normal whey, except u will burn a bigger hole in ur wallet.

Preworkout? brown rice plus beef or chicken, or oat and eggs.

Yes u can, if u want to gain muscle, real food still playing a enormous role in it, the calories in whey powder is alot lesser than a piece of beef that has the same amt of protein.

is actually better