If you enter just one item for each part, don't enter a part number specific to any one manufacturer.  Also, if the pricing is different don't enter a default cost either.  You can enter the cost directly on your purchase orders, bills or checks.  Just remember, QuickBooks uses average costing to determine cost of goods sold.  So if you buy the same part from one manufacturer for $5 and another manufacturer for $6, your cost of goods will be $5.50. 

Depending on the level of detail you want on your reports, you can add separate sub-accounts for revenue, cost of goods sold and/or expense.  Just go to Lists > Chart of Accounts, click the Account button, select New and select the appropriate account type.  To make a new account a sub-account of another, click the check box and choose the parent account in the drop-down menu.  It should end up looking something like this:

Revenue:
   Parts
   Services

Cost of Goods Sold:
   Parts
   Services

Alternatively, you can uses classes for parts and services.  In this case, you’d keep just the default revenue and cost of gold sold account and create classes for Parts and Service.