Depends on what you're prepared to pay, sorting the problem is cheaper than rebuilding. If the headroom will stand it put 100mm-150mm of concrete down, this should stop any ingress through the door way.
Either use a membrane, or my preference, water proof concrete.
If ground water rises up the walls inject a silicon damp proof course. Silicon can be done on a DIY basis to save paying a professional.
Gutters and down pipes will help prevent penetrating damp, if the ground remains waterlogged, and you wish to prevent that, then drainage is really your only option.
If you do it all yourself, then yes, it is damned hard work.
To achieve this at in my place has involved digging paths to a spade depth and 5ft wide all along the back, a dry stone wall, 25 tonnes of shale, all wheel barrowed into place, and 15 tonnes of 40mm gravel as topping.
Prior to this work our standard foot wear once we stepped out the back door was Wellies!
Roy.
Mind you I was 18 years younger then!
In Wales of course it don't half rain!