I hazard the guess that what you are looking for is already available for livestock monitoring, so I would look in that direction unless you want to re-invent that wheel. I admit I am not 100% sure what is out there as I am working on something unrelated. Homemade tags may be cheaper but don't underestimate the effort and experience required to make something reliable and robust.
Reliably monitoring body temperature and heart rate may also be more difficult than you expect. I looked into this some time ago for birds such as golden eagles and discovered that the most reliable option required attaching a sensor onto the skin. I then decided that I would have to do without this and moved on. I am in no way an expert on this and others may know much better so don't give up quite yet. Again, there may be ready-made solutions in the livestock tracking market that do these types of measurements already.
Anyway, I can offer you a few details on GPS components I have come across that may be of interest. Please note that although these components are on my tracker prototype, this is still under development and I cannot yet say how well they perform. But their technical details looked good enough to give them a try for my project.
GPS MODULE: uBlox EVA-M8M
- certainly not the cheapest, GBP 10 bought in single units, cheaper in bulk;
- stores GPS satellites' almanac and ephemeris data onboard and can interpolate future trajectories. This means that the module has to be switched on only once in 48 h to get all the satellite data (typically takes 45 s) and then fire up only for a few seconds to get a position fix. The key benefit of this is energy conservation, which on most tags is important.
Data sheet: https://www.u-blox.com/sites/default/files/EVA-M8-FW3_DataSheet_(UBX-16014189).pdf
GPS ANTENNA: ProAnt PRO-OB-430, GBP1.00
A very small and lightweight GPS antenna, much more so than the usual ceramic patch antennas. If weight and size are an issue then this may come in handy.
data sheet: http://www.proant.se/files/user/Application%20note%20-%20OnBoard%20SMD%2...
A hint for more inspiration and sources of useful information on lightweight and energy-efficient electronics: websites on remote controlled model aircraft and amateur weather balloons.
It would be useful if you could post here a summary of your results on the GPS module search, once complete, for the benefit of those looking for the same details in the future.
Good luck !