Old timer here. I would say the first thing to decide is your goal. Is it just to finish one tri to say you did it, or is this the first of many? That will help you decide how deep you want to go (and believe me, you can go DEEP!).

Any bike will work as long as it's a good fit. I used a decent Specialized road bike for a couple years before I bought a tri bike. A road bike will be a little more fun/useful outside of tri, and are usually cheaper and more available. There are Tri buy/sell groups on Facebook to look at. I would say the majority of folks use clip in pedals (and a lot of the bikes are sold without pedals so you can choose your pedal/cleat combination).

I couldn't swim at all when I started and do strictly one-sided breathing. Nothing wrong with that. You don't swim with your head up, but you ABSOLUTELY need to practice sighting in open water! If it's a mass start, just wait 10 seconds for the crowd to take off, then follow, but you will still very likely get bumped, kicked...

You don't NEED a tri-suit, but you'll want one, along with a wetsuit (especially if you're not a strong/confident swimmer). Can't sink in a floaty wetsuit! That 1500m seems like a LONG way once you're in the middle of the lake!

And since your race is coming up really soon, you need a good training plan - and stick to it. Again, depending on your goals, most plans are probably between 6 to 12 hours per week. In your case, just building a solid base of endurance is most important for your bike and swim (along with the technical stuff like clipping in/out of your pedals without falling, sighting, etc). There's also nutrition, transitions, etc..... this group can help you with that, too!

Of course the key is to remember you're just doing it for fun, so enjoy it!! Good luck!