Technically whether limited company or SP, you still need a BR, legally speaking. Fees are 2250/year or 5950/3 years. For this coming year there will be a discount so it's only 250/year. Audit costs should run you 2k per year. You can spend more if you don't want to shop around. Deregistering a company is a process and expense as well so most keep it active.
People usually only sue if they think they can get something from you. Otherwise it's a waste of time and energy and possibly that you don't ever pay the debt. So most starting out companies can just try until they feel it is worthwhile to make a company. The decision is ultimately yours. You can quickly burn cash if you set up every process that developed companies have. Making the right decisions at the right time is key.
Additionally you can obtain professional insurance, which I have to protect the company in the event of a lawsuit. I did not have this on day 1 but after I found the idea would stick.
If you only have small revenues let's say under 100k hkd per month, I wouldn't bother to set up anything. Focus on revenues then play catch up on processes and procedures later is a better way, in my opinion.
In some cases a customer will only work with a registered company or only pay to a legal entity (not an individual). Then you'll have time make the decision of what to do. In HK it is not cheap per say compared to other markets and it is an ongoing cost so make your decision wisely if you are in the game or not.