What a shady company, they don't want to follow the rules of Hong Kong. Sounds like trouble...
Anyway, if you need a visa, you can't do this freelance or contractors status legally.
You need an employment relationship first. Without it, just forget it. You get little rights as it is as an employee so they shouldn't try to take that away.
For immigration, once you have an employment contract, you complete forms 990A and 990B and show proof of your university degree.
You pay tax, the employer does not pay taxes because of you.
Both parties (employee and employer) each pay 5% of your monthly salary, up to a maximum of HK$1500 per month, into a retirement fund called MPF.
You need a notice period, if you or your employer wants to terminate the contract, you shall deliver a notice or payment in lieu.
Standard terms. It's literally all right there on the internet and HK immigration and labour department website like said above.
I've dealt with companies like this and passed up the opportunity to work with them, because if they are doing this, they have no idea the way things work here and I'd rather work for an established and stable company versus one who is trying to cut the basics. If they can't get this right, what faith do you have that they will get your salary paid, bonuses, commissions, etc.