How to search the workspace?

You need a recursive function -- that is, a function that calls itself.

function search(where, what, results)
		--where: the desired model
		--what: a function that returns true if it's what you're looking for
		--results: you don't need to provide this; it's the table of results
	results = results or {}
	if what(where) then table.insert(results, where) end
	local ch = where:GetChildren()
	for i = 1, #ch do
		search(ch[i], what, results)
	end
	return results
end
function printResults(results)
	for i = 1, #results do
		print(i, results[i])
	end
end
--example of usage:
local results = search(workspace, function(obj) return obj.Name == "Special" and obj:IsA("BasePart") end)
printResults(results)

Inline explanations:

function search(where, what, results)
	results = results or {} --initialize 'results' to an empty table if it has no value
	if what(where) then table.insert(results, where) end --if the custom function 'what' specifies that 'where' (some object in the workspace) is what we're looking for, add it to the table of results
	local ch = where:GetChildren() --get the list of children of 'where'
	for i = 1, #ch do
		search(ch[i], what, results) --recurse on each child
	end
	return results --return the table we collected
end
function printResults(results) --this is just a function to print out the results we found
	for i = 1, #results do
		print(i, results[i])
	end
end

--Now, in the usage example, we have a call to "search":
search(workspace, function(obj) return obj.Name == "Special" and obj:IsA("BasePart") end)

--Broken down:
search(
	workspace, --where to search
	function(obj) --'obj' is something in the workspace that 'search' is considering
		return obj.Name == "Special" and obj:IsA("BasePart") --this means "return true if the name of the object is 'Special' or if the object is of the class "BasePart". You can change this line to serve your needs.
	end)