Thanks for sharing your enquiry. General rule of thumb will be that as the part-time income is irregular, financial institutions will apply a 'haircut' as you mentioned, but it is up to their assessment factors such as the nature of the income and how regular does it come up and the applicant age profile etc.
Supposed in a conservative scenario they can recognize 8k/mth for the household, based on current MSR regulation of 30% and max 25 year loan tenure, the maximum bank loan attainable will be $479K.
Say for a 800K EC purchase price,
The balance $321K will have to be met by your combination of Cash (minimum 5% of purchase price), Combined CPF OA + your housing grant of 30K. It is advised you speak to your preferred banker to get an In-Principle Approval of the loan amount so you can have an idea of the comfortable pricing range of the units you will like to look at.
To address your 2nd question, for North-East region, there are presently 2 ECs in Punggol (Ecopolitan & Waterwoods). Another upcoming 3 (one in Sengkang, Bellewaters, another two in Punggol). Of the current 5, Ecopolitan is the nearest to the mainline MRT network, which is Punggol MRT.
The tendered land prices for the upcoming launches are higher (which typically translates to a higher launch price) but they will be differently themed from the current ones.
May I speak to you to understand your housing requirements further? I will be glad to meet you in the North-East to share more about the pros & cons of each projects, as well as the latest starbuy prices of the ECs that are launched and launching soon. Thanks and hope to speak to you soon!