I'm having new shingles put on a duplex and the roofer has two options:1) BP Mystique 42 Limited Lifetime Warranty Shingles  (base shingles)2) Malarkey Vista 40 yr Warrant Shingles (+$950 upgrade)Roofer figures upgrading from base shingles will extend shingle life from about 20/25 to 35/40 years. 

However, insurance is limited because it only protects you from one type of liability: accidents/negligence.

But we are not sure if we can find a good mutli family in the area within the time limit.

I love the strategy of small multifamily properties. 1 roof, 1 foundation, multiple tenants :)In the Charlotte market, this strategy is VERY challenging, just because the inventory is much more limited than single family/townhouse/condos.

Here is where it stops I shall tells yous.You set your limit BEFORE you even negotiate OR try to convince yourself a bad deal REALLLLLY could be a good deal if you tilt your head a bit and look at it from an other angle.If 29K is your limit, you walk if someone wants 29.5K for it.This is a system.

Limited area. 

@Geoff GarberAs several people have mentioned here, and as the Russian proverb entails, "measure it seven times, cut once!" 

Perhaps one with strong real estate knowledge, but also one who can advise my wife and I on a variety of other topics, including but not limited to her 401k, stock investments, college funds/trust funds for our 5 children, etc?

However, with limited funds of just 20K and with minimal construction experience the single family is easier starting out.

IMO, that's the base case you should be measuring against.