In order to enter certain race/combat events, or to travel/trade between towns in the game, you will need to purchase vehicles for your gang. Vehicles are bought from the mechanic shop, using the game web site (click here for a description of the Town Interface). Note that stock levels (and prices) are dynamic and based on supply, demand and piracy. New stock arrives at the mechanic shop at different times of the day and night.

You can perform four different actions at the mechanic shop:

Order a New Vehicle - design and order a brand new vehicle from the mechanic (see below).
Buy/Sell Components - trade engines, weapons, and tyres with the mechanic. These items transfer between the shop and your town lock-up. This lock-up is effectively a staging area between your actual vehicles and the mechanic shop: in order to get components from the shop into your car (or vice versa) you must go through your town lock-up. You can store equipment in your town lock-up for as long as you wish, but be aware that there is a risk of burglary each night. The risk is quite low in relatively safe towns such as Somerset, which are patrolled by numerous vigilante groups.
Install/Remove Components - these components transfer between your town lock-up and your vehicles. As well as installing a component for use, you can put components in your vehicles' cargo areas (assuming they have enough space).
You will see a list of vehicles that you currently have in town, and by clicking on one of these you can perform maintenance work on them in the mechanic shop (see picture on the right). The actions that you can carry out are coloured brown: in the example on the right, we could modify the amounts of armour, repair and/or reload the damaged machine gun, or sell the vehicle to the shop. If a component's health is less than 75%, then you will not be able to repair or reload it if the mechanic shop does not have at least one item of that type in stock.

When ordering a new vehicle, you have the following components to choose from:

Chassis
Engine
Weapons
Tyres
Armour
Fueltank
A vehicle's chassis determines its roadhandling, inherent toughness, storage capacity, engine capacity, and a variety of other factors. A table of chassis types is available here. A vehicle's engine affects its speed and acceleration; each chassis has a maximum engine size that it can take. Weapons come in various flavours: melee, rocket, projectile, dropped: see here for a table of weapon types. When choosing your vehicle's fueltank size, you need to consider whether it will require to travel long distances. As a rule of thumb, a 1 litre engine does 10 miles per litre of fuel. Since each 'bulk' unit in a fueltank stores 2.5 litres of fuel, a tiny fueltank (2 units) with a 1 litre engine gives 50 miles of travel.