1. Please stop stressing out about everything. There is still long time until you leave, and there is no need to book everything now.
2. There is absolutely no risk that all January hotel rooms in T1 will be booked if you contact them in September or October. For the hotels in T2 and T3, the hotel operator has this info on their web page: "It is recommended to make your reservations 1 to 2 weeks in advance." Since the demand for the T1 hotel should be about the same as for the T2 and T3 hotels, the January hotel rooms in the T1 hotel won't be sold out in September. Once the refurbishment is finished, reservation info and room info will be available here.
3. Pricing for T2 and T3 hotels is available here. The rates for the T1 hotel will probably be very similar.
3. Instead of getting hotel room, perhaps you can go on a free city tour and visit the attractions in T1. Your transfer time isn't that long, so a city tour + spending some time in the cactus garden or the kinetic rain would probably use up most of your layover. Then you can sleep on the flight to HEL so that you are well-rested when you arrive.

Why? The hotel is airside, so all you have to do is walk from the hotel to the Finnair gate. There is absolutely no need to be at the gate two hours before departure.