Running HW baseboard

Crawdad20

I have a bed/bath addition that I need to get heat into. I currently have 2 zone oil fired hw baseboard (upstairs /downstairs) with a 3rd zone to a boiler mate  for DHW.  The plumber that did the bath rough in gave me a price of 5K to add a third zone and heat to the addition (16'x23' , right next to boiler easy access).  I'm not a cheap skate or lowest bidder kinda guy but I just don't see that much work there ( and I don't have 5K LOL).  I'm thinking about giving this a go myself and have a few questions.  I'm sure a separate zone would be more economical in the long run but is there any reason I can't tie this addition into the current 1st floor zone? If I did the entire loop would be about 175' including heat elements. Total footage of baseboard would be about 65' if my heat load calcs are right. If that is not a viable solution can I just tee off the first floor supply side with a shut off valve and tee back into the return side with a zone valve below the zone valve for the existing 1st floor zone?  Additionally, I'm thinking about making the switch to pex. I'm confident in my soldering skills but there' lots of tight spaces in this old dry wooden house. Any recommendations on brand of pex/tools/connection types?