sorry for delay
this is a made up answer murder and drug dealing are more my thing:
1 search on land reg make sure it is the vendors land to sell and not subject to a mortgage or other debt, few quid
2 local search make sure local authority don’t plan to knock it down etc £60 £100.
3 other “local” issues eg coal mines cant imagine an issue re a shed a house i owned had a pit shaft covered over nearby.
4 fee to register your ownership £20 to £40 depends if the garage is a whole registered title or is being subdivided off.
taxes as many have pointed out on here i ain’t financially astute stamp duty 0% or 3% if btl or additional home rate. no vat.