Hiya,
Nice project, wrong budget, mostly wrong selection of parts.
Depending if this is a one-off or something you plan to manufacture in qty > 2, I would have different suggestions for the hardware.
In either case, PIC is ancient and using ENC28J60 is dumb because pretty much any modern micro has ethernet MAC already built in.
I'd suggest using something like STM32F105 (or 405, or anything with Ethernet), these chips have plenty of hardware timers (PWM), I2C bus ( for proper temperature sensors ), plenty of ADC channels (in case you insist on using analog LM335Z parts ), etc.
If you're just doing a one-off there's some STM32-nucleo boards with ethernet on them already that you can make a shield/topboard with the sata power/fan/sensor sockets and that would probably be the most cost-effective solution for a one-off board (nucleos are around $20-30).
I'm not really sure where you're getting "This is a pretty simple" project, when you're talking about taking some ancient stuff from nearly decade ago, using obsolete parts, switching it to ethernet control, swapping MCU to something completely different (because who the fuck uses PIC in 2017), no, it's not simple.
The budget is also too low, even my bid is too low but freecancer doesn't allow bidding a lot over original budget, I'm going to guess this is around 2k worth of work including prototype and firmware development.