If you plan on keeping the boiler I would do the hydro air as @Rick Bassett mentioned with the air handler in the attic if possible to supply the heat and A/C. 

As far as San Francisco (and New York, LA, and Miami) goes, I just would like to add housing is always supply vs. demand, of course, but it historically it is correlated with jobs and medium household incomes.

The BEST option would be to replace it ALL as well as any metal supply piping cause its time. 

It's a nice gesture to supply a fire extinguisher under the kitchen sink or maybe one in the garage for safety, right?

To keep the hard cost down somewhat I DO plan on doing some Serious negotiating as far as supplies and labor goes .Not that I am a cheapola but there are bargains around if you shop hard enough .

If I am in a partnership where my partner owns and supplies the house and I supply the renovation budget.

Yes, it's possible to make some money here but the market is super hot, a lot of investors here have extremely deep pockets and the demand is out stripping supply by a wide margin.

But if you have three amazing candidates and one wiling to pay it, then go for that one.I'm not sure how the supply / demand is within your area at this moments, but this gives you more qualified demand to select a qualified tenant. 

Makes me think it's not a supply or supply-line issue, but I'm certainly not a plumbing expert.Any thoughts?

But, remember, the broker needs to make his/her money somewhere, so if you're not paying commission splits on your transactions, you're probably paying much higher monthly fees.A broker incurs a lot of costs from the agents s/he oversees -- including liability, training overhead, office supplies, paperwork review, continuing education, etc.