Just have him pull a Dedicated 20A, 12/2 grounded circuit up there to a switched 4X box..tell the electrician you want to plug in a light and add some fans to ventilate attic heat in the summer, or better yet say nothing and just tell him..or her... what you want. If he says a 15A will do, just tell him..or her..you would just prefer a more robust circuit...someplace to say..plug in christmas lights..Maybe he'll have a razzzu timer for those..
You should end up with a 4X switched outlet (the funny looking one like you see in the hospital). Now.. a 30A with 10/2 grounded would be better but its more expensive, plus leaves a ton of room for growth. 20A will be plenty tho. The 20A will be good for @2k watts of load give or take 2-400W. The 30A will be good for @ 3K. which is A LOT and a good electrician will start asking silly questions..It should cost @ 200 and parts. Circuit breaker, wire, box, switch, etc.. another 100 or so depending on the price of the wire. 300 dolla? double the parts cost on the 30A stuff.
I would not even consider a 15A undedicated, In my humble opinion.. Being undedicated I think its a little light and can be less the reliable. If its not dedicated, enter extension cords..it will only take once or twice of the breaker tripping when you turn on the vacuum or microwave before you consider hard wiring your circuit.
If you do choose to use a drop cord spend the money and use a good one..12/3 or even a contractor grade 10/3. Find the circuit in your place with the least use and tap it. Keep in mind everything from your cord to the panel is probably sized for 15A tho. NEVER undersize your wire or mess with switching breakers..ever..your house could burn down.
I'll share two things then shut up:
1. The circuit breaker is protecting the wire downstream that it is rated for. IF you run a CB that is rated higher than the wire you are protecting you will overheat an undersized wire. result..fire. To figure your wire/breaker size add up all your loads that will be on the circuit..multiply that by 1.25..Example 1: A 600W light load, a 100 watt fan load, a couple pumps @ 75 ea.. is 850W. Multiply by 1.25..gets you just over 1K.. 15 amps is plenty. (about 1700w to be more precise) Example 2: 3x600 watts in lighting, maybe a few 37W cfl's maybe, add 200W in fans, you are well over 2K and will be liking the 20A circuit.. PS>>when adding up your light load, use the ratings on the Ballast.
2. Stealth is cool, but its hard to invent stories for cords poking through to the attic. I saw a guy with a fricken hose poking out the front of his gable just not long ago..lol. A big A$$ green garden hose..but hey, its not illegal to have a hose running into your attic..is it? It just raises questions..
You are wise to hire an electrician if you can afford it, very much worth the peace of mind. Happy safe growing.