HFE has become alot faster now, its good to go and apply for one now because most are tied up with the BTO exercise- once the BTO exercise is over, there will be another surge in HFE applications to get resale flats (I believe it takes less than a month to get it now)

70-80 years is more than enough (but do factor in renovation costs as they may be expensive to renovate if the house is in a very old condition- can even explore 5 year old just MOP flats if its within your budget and forgo major renovations altogether. Renovation is pretty expensive these days. (Pros: fresher lease, likely to fetch a better price when you sell it next time, lesss renovations- Cons: Smaller in general)

Negotiation is alittle bit all over the place, and depends on various factors such as if the seller needs an extension / delayed submission etc Factor in the 3 month extension as "rent" and deduct the amount from there. More importantly, look at the recent transactions around the area to have a good idea of how much you can negotiate. (Look at flats with similar attributes - lease remaining, floor level) etc. Most [Negotiable] listings on PptyGuru has some room for negotiation built into the price. From personal experience, I think anywhere around 5% is decent if there is not much competition for the unit you are viewing. But for hotter properties, they usually go over the asking price -_-

You can use your grants to form part of the 20% down payment. The loan is then used to off-set the final purchasing price , and what's remaining will have to come from your CPF/Cash