Before offering to buy a plot you are advised to appoint a local surveyor to undertake a comprehensive assessment – see the SBE Site Assessment Checklist to establish its suitability for your purposes.
Having found a plot and agreed the price you need to instruct a solicitor to act on your behalf to submit a suitable offer, subject to planning permission, to the vendor’s solicitor. Do not be tempted to seek cheaper options. It is important you have an independent advisor who is not only expert in the relevant legislation but also bound to protect your best interests. NB Most solicitors in England and Wales will not have the expertise to act in Scotland and vice versa.
Plots are often offered with Outline Planning Permission (Planning In Principle in Scotland), but this only establishes the principle and will likely impose certain conditions. You still need to seek full (detailed) planning permission for the siting, design and servicing of the actual house you wish to build.
If the land is part of a larger holding you need to ensure you get a title plan showing the land in relationship to surrounding landmarks and boundary positions / dimensions. This is particularly important where there are no fences or other clear boundary features such as hedges and ditches.