You want a job in investment banking and you're thinking of buying a cheap suit in Thailand from Rajawongse? I'm shaking my head in bewilderment. But then again I guess for someone who doesn't know any better and wants advice, you did the right thing in getting opinions before taking action. My advice is to totally forget any tailior in Thailand. There's no good deals to be had. Any cheap suits will be exactly that - cheap. They'll either look horrible, not be cut the way you want and/or not last very long. Of course if you've never bought a suit before and don't know well about how a well-tailored suit should look and last, then you'll probably be satisfied out of ignorance. Based on my experience there's about a zero percent chance of someone inexperienced in suits going to one of these tailors in Bangkok and being able to get something suitable for wearing to a job in investment banking. Then again you don't want to spend over $300, so you don't really have any good options.
I know lots of people swear by their favorite tailor in Thailand. I'm sorry, but I simply cannot agree that there's any quality in ANY of the typical tailors the tourists go to, including Rajawongse, and if you go to one who does have some quality it'll cost you much much more and won't be constructed as well as a custom made suit from a reputable tailor in the west. If looking to have a suit custom made in Asia, I'd only be looking at Hong Kong myself.
I suppose though if you really can't spend the money for a decent suit and just need one for some job interviews, and your age/resume reflects that you're an entry level, then go for it. Just be prepared to replace that suit soon though after you land your job.
If you want some good advice about clothes and tailoring, this isn't really the place to be asking. I'd suggest the Ask Andy forums or something similar:
http://www.askandyaboutclothes.com/forum/index.php
Given the subject matter of this forum, you have no way of qualifying anyone's opinions of clothes. Some may know what they're talking about but 95% are no better than asking a random man on the street. Seek out advice from people who know their stuff.