Crikey, this is a big warning signal - I can hear alarm bells ringing from here!
1) Not a good sign. A builder that cares would not want to subject his customers/purchasers to any more nuisance from the building works other than that which is unavoidable.
2) It is an NHBC requirement that garden areas are topsoiled and free from weeds. If the builder has left it so weeds are 6ft tall, clearly the home has been on the market for quite some time and he doesnt care at all about presenting his homes in the best possible way. Turfing or grass is not normally included to rear private gardens.
3) Not all carpets have underlay. The cheaper carpets (and this is probably what you have got going by what you have already said) is foam backed and only lasts about 18 months!
4) The skirtings can be filled if necessary and the decoration touched up as part of the snagging process. The carpets should be cleaned or replaced before occupation.
Finally, this is NOT USUAL. My advice is buy a house somewhere else. This one is going to be nothing but trouble for you.
If you still want to proceed, Do not Exchange Contracts unless you are happy to do so AFTER a professional snagging Inspaction has been carried out.
I would recommend Brickkickers.