Help getting started...with everything!

The 120km is do-able but tough IMO. You might find the difference between the 80km and the 120km is that you finish the 80km happy and you suffer the last 40km on the 120! You'll have a much better idea nearer the date.

This time last year I went from being taught to ride a bike to a very hilly 105km sportive in 4 months. You've a month less but you can already ride a bike.

Buy a road-bike - that's a bike with drop-bars like you see in cycling on TV, not a straight bar like on most town bikes. A road-bike has 2 positions with full access to brakes and gears. A straight-bar has only one. The road bike then has another 2 positions (at least) where you can't access gears or brakes but these are fine for brief periods on steep climbs or quiet safe roads. The extra hand positions means your arms, shoulders, neck and back all get to move around and don't spend the entire time in the one position. Which makes all the difference for how long it takes them to get sore.

Some people can do 100+km on a straight-bar hybrid, I can only do 30km before I start hurting.

Most people who don't know road-bikes think the drop-bars look a bit scary and they do feel strange at first but you get used to them really quickly.

I've had good experiences with Think Bike in Rathmines (http://thinkbike.ie) and they let you try out a bike for a couple of days to see if you like it before buying it. I've not seen that offer elsewhere. There's other good bike shops out there, I just happen to have personal experience of Think Bike and can recommend them.

As to how much they cost, how much do you want to spend!?

If you can use the biketowork scheme you can get up to 52% off depending on your tax bracket.

You'll get the best new-bike value for money buying online, there's some great deals. Plenty of threads on here listing them. I wanted to buy from a bricks-and-mortars store as I wasn't confident enough about bike maintenance at the time. I am now.

You'll get a really nice bike for a grand. Or a perfectly nice bike asking 625 - http://www.thinkbike.ie/bike_shop/2014_Giant_Defy_5.aspx . If you're not using a biketowork voucher company (who take a middle-man-percentage) then you should be able to negotiate a similar percentage off the asking price IMO. 10% is a reasonable target. You can get that bike in most bike shops. You can get an ok bike for less again - there's a Carrera TDF model that a lot of people consider good value. Around 400 euro from Halfords during their regular sales. 2nd hand bikes offer better value again but I was too unsure about bicycles at the time to buy a 2nd hand bike. There's lots of very good brands - Giant, Trek, Specialized, Felt, Cannondale and plenty more. Post up what you're looking at here and you'll get opinions. Or ask in a store that you have a decent recommendation for.

You'll also want a good lock and be really careful where you lock a bike. Cable locks are all ****. I wouldn't use anything cheaper that the kryptonite evolution mini.

If you'll end up commuting to work then getting something that can take a pannier rack is really nice. I hadn't realised how handy it'd be but I now regularly bring home 30 euros worth of meat from the butchers near my work place, comfortably carried in the pannier. Backpacks are annoying ways to carry anything on a bike. Sweaty and uncomfortable.

Mudguards are great.
Lights are necessary.

Get out as often as you can, within reason... A lot of people find that they really enjoy cycling and want to get out and the problem at the start may be stopping yourself from going out too much as your body (especially your arse/legs) will take a bit of time to get used to the bike.

Oh yeah, buy some bib shorts with a decent pad. They make a huge difference on long cycles.

How hilly is your planned cycle? If it's hilly then you'll want to start doing hilly cycles in about 2 months time.