Decisions, decisions

We are looking for opinions/thoughts on our financial situation so as to make a decision on our next move towards financial independence and to stop sitting on the fence. Any and all help would be greatly appreciated.

Our situation is this:

We are are married couple in our early 50’s – self employed builder.

We own the following:

Mortgage free
Residential home $350,000
Coastal holiday unit $190,000

Mortgaged
IP coastal property $200,000 (o/s mortgage $40,000 returning $160.00 p/w gross rental) $220.00 weekly repayments with $20,000 in MISA a/c to reduce interest.
IP block of land $150,000 (o/s fixed 2 year mortgage of $100,000 with weekly undeductible payments of $160.00.
Have total of $100,000 in internet acc/shares/managed funds plus $315,000 in super.

Question:
Is is wise to borrow total costs of say $150,000 to build a house on investment land taking mortgage to approx. $250,000 in total for property, then rent it out (negatively geared) or sell creating CG tax. Should we sell some shares/managed funds to fund part of the construction. As loan on land is fixed for another 18 months we cannot pay extra on this loan.

We would greatly appreciate constructive views on this situation.

[confused2]