To install a mains pressure HWC will cost around $2500+.

Depends on size, valves etc needed. If any carpentry needed too  - sometimes the cupboard won't fit it, the sizes are different these days.

Gas, well expect more cost.

Dribbling...meaning what? Difference in hot water versus cold from shower?

 

Or you just think it isn't coming out fast enough?  Remember with mains, your water usage will increase.....