Will you wear a pointy helmet? They are worth more speed than TT bike and considerably cheaper.
Can you hold the TT position for 6 hours? (flexibility and fatness)
Can you afford two bikes?
Do you expect to ride over 7-hours? (could be embarassing on a TT)
I would rather spend my money on a decent road bike first than a TT which is less comfortable, less use, more maintenance etc.
M..eface