Couple questions regarding sump building and operation

Not if it's the only compartment. You can hang your current HOB skimmer on the return section too, so you can keep it and have the extra space in sump for more fuge.

 

Melev has a very good explaination on sumps found here:
Melevsreef.com | Acrylic Sumps & Refugiums

Baffles are an important part of the sump as they help dissipate microbubbles caused by skimmers and the water falling from the overflow plus provide a fairly constant level for insump skimmers to operate properly. Other benefits are they help particles settle out due to the changes in direction of the flow and they provide more volume as in the soultion to pollution is dilution, the more water volume the steadier the tanks water parameters will be.