This book has a table with the energy budget for Huygens:
Data relay system: 214 Wh
Computer: 246 Wh
Electric power system: 5 Wh
Payload: 325 Wh
Losses: 132 Wh
Pre-separation checks: 50 Wh
Total: 972 Wh for the designed mission:
The batteries and all other resources were sized for a Huygens mission duration of 153 minutes, corresponding to a maximum descent time of 2.5 hours plus at least 3 additional minutes (and possibly a half-hour or more) on Titan's surface.
Then they added a big margin to account for failed components, and arrive at a capacity requirement of 2059 Wh.
Thanks to this margin, the probe ended up functioning for 70 minutes after landing.
According to this timeline, Cassini dropped out of sight of Huygens just over 2 hours after landing, giving another constraint. It would take a while before Cassini came into view again (how long? I don't know, but IIRC Cassini's orbits were on the order of a month), so your design decision is to either size the probe for 2 hours, or for it to survive until communications are reestablished. If that's 1 month, you can't carry enough batteries to survive that long.
There were no electric heaters AFAIK, Huygens carried radioisotope heaters.
The major constraint is mass. Huygens' total mass was about 300 kg, and all the systems, experiments, structure etc. must fit in that mass. You can't increase the mass by much without jeopardizing the launch (IIRC Cassini was already the heaviest interplanetary spacecraft).
Work on Cassini-Huygens took place 1988-1997, which places constraints on the technology you can use: e.g. computers take a lot of power, limited battery chemistries available and proven for 10 years in space.