For me to go from being just a runner who occasionally cycled a bit, to doing The Longest Day ('ironman'), took:
- wetsuit - donated, but can hire cheaply as Barlist said
- swimming costume
- goggles
- silicone hat - wearing 2 keeps your head warmer. Put talcum powder inside these after each use, or they die quickly.
- I already had an adequate bike (contrary to what everyone thought on the day, it is NOT a 'mountain bike') - but with flat handlebars & no cleats. This was not a problem for me but some may disagree!
- Other people also seem to like cycle computers (£20-30 will get you a decent one, and if you buy it from a proper bike shop they will fit it if you ask).
- For training purposes you may also want bike lights, in case you are stuck out after dark.
- A bike repair kit, best kept in a seat wedge, and 2 spare tubes, & a lightweight frame pump (unless you muck around with CO2) are essential. Add to the bike stuff a packet of jelly beans or spare fuel of choice, ibuprofen, a bit of money, a 20p coin and maybe a mobile phone. If you can jam it in an ultralight windproof has been very useful for me.
- If you don't have a helmet already, you should get one. Any BS kitemarked one will be adequate for crash protection, what you pay for is more holes & graphics (I now work in a bike shop, mostly on the strength of TLD). £20-£30 should get you a decent one with enough ventilation - much above £30 & you are paying for looks & gramme savings which could be better made elsewhere.
- Extra bottle holders for bike - essential but cheap.
- Bike top - this is the only bit of clothing I brought specifically for the event. These have 3 really big pockets on the back, for malt loaf & puncture kit.
- Bike glasses - useful even in dim weather for eye protection. Again, more than £30 is spending on style above function.
- Clothes wise the other things I wore were a standard running sports bra & lycra shorts (right through - including under wetsuit), & a club vest for the marathon, & 2 pairs of socks (dry ones for the run).
- if you are already a runner you don't need any more kit for the run.
I spent about £100 all told on everything except the entry, including things like swimming tickets & extra food (veggie jelly beans). I did scrape by on minimum kit & got a few things donated (thanks - you know who you are!), but it does not have to be a ferociously expensive sport if you don't want it to be, though it is very very easy to be persuaded into buying lots of things you don't need, or where a cheap version would do just as well.