Firstly, to readers, this is a bike buying guide for triathlon specifically. I'm certainly no industry expert, but hopefully I can help those looking to either upgrade or get their first bike.
There's so much choice out there, it's very difficult to start somewhere. Often it will be what's available in the local bike shop, but with the internet at your finger tips, the best deal is most likely going to be online. I think another important point to make is not too be afraid of the smaller sellers. I believe people tend to stick to the well known stores such as Wiggle or Evans, and don't shop around.
1. Road Frame or TT frame.
Your flexibility - those struggling to touch their toes might not be too comfortable on a TT frame. Stick to a road bike for a more upright posture. Comfort is king. You can't put out maximum power with a sore back.
Events you've entered / expected use - A lot of triathlon races tend to be on flat courses, which suits a TT position and therefore a TT frame. If Alpe d'huez is more your kind of scene, its a light road frame you'll want.
Price - Generally speaking, a specialist TT frame or triathlon bike is more expensive than a road bike. However, high end road bikes are similar in price to a lot of TT bikes nowadays. For second hand bikes, prices fluctuate wildly and there are many stories of people picking up great bargains from Gumtree or eBay. It is possible to get a great price, but think about warranty and durability. For example, it’s fairly easy to cover up damage to a carbon frame, so be careful what you bid for and always try before you buy.
2. Carbon vs Alloy
If you’re budget allows for it, get a carbon frame. They are stiffer, lighter and look A LOT better. You’ll always appreciate having a lighter bike underneath you. Not only easier to climb on, but they have much more responsive for steering purposes, and the increased stiffness means more of your energy is going into actual speed.
Having said that, carbon can degrade is harsh weather conditions. Riding around a lovely new carbon bike through the British winter on salty roads isn't the best idea. If you clean it thoroughly and regularly then you'll be okay, but perhaps stick to an alloy frame if you know you're not going to keep up with the cleaning regime.
There are a lot opinions about what brand to go with, but stick with Shimano. Most pros are using it, and aside from sponsor commitments, there’s a reason why. It has long proven functionality and it just works! The mechanisms are un-complex and most maintenance can be done at home. Add to that the worldwide usage of their components - pop into any bike shop in the world and they’ll be able to fix/replace Shimano components, other brands you might not be so lucky.
Electronic vs Mechanical - Like most of your choices this comes down to a question of budget. Electronic ‘groupsets’ (The whole component bundle - gears, crank, brakes e.t.c) are easily double the price of a mechanical groupsets. Half the price of a decent bike with electronic shifting can be the gear mechanisms. Personally, the performance of mechanical shifting is almost indistinguishable to electronic shifting. There is a sliding scale of Shimano componentry however - the more you pay, the better the performance. It starts with Shimano Sora and goes up to Shimano Dura-Ace. In the middle you’ll find Shimano 105 and it offers the perfect balance of performance vs price. I would highly recommend.
I haven’t yet mentioned the advantages of electronic shifting on a TT frame. With most electronic groupsets, the front and rear derailleurs are operated with little ‘blips’. Using these instead of the traditional levers (as with Shimano 105) is much simpler, and it also allows you to change gear whilst sat up, out of the TT position, with your hands on your brakes. If your budget can stretch, Shimano Di2 (electronic) cannot be beaten. So save up your pocket money.
It’s a fine balance, finding the perfect frame and components. You can find a beautiful machine that just doesn’t have the right components or visa versa. Both frame and components have to be taken into account when buying. One doesn’t come before the other, which can make the decision really difficult. Although a hassle - it is worth knowing that you can always upgrade your components at a later date, if your budget is limiting you at the time of purchase.
4. Bike fit and extras
Some people swear by a bike fit, whilst others choose to fit at home. I think the value of a bike fit would come a few months into you riding your new bike, when you want the position nailed. You need to get used to the bike and find what’s comfortable, then go back to a specialist to have the position fine tuned. An initial set up is easily done on a turbo trainer, and make sure to match your old bikes measurements. This is also important when choosing the frame size. Most manufacturers will take your current measurements and suggest the best frame size (You could then take this to find a second hand bike in the right size). Again - try before you buy! Bikes are very much like running shoes, not all manufacturers have the same sizing guide.
Make sure to have some allowance in your budget for added extras. Whether this is pedals, a new helmet, bike lights etc, don’t forget to include them. Some bike shops will often throw in some extras when you buy a nice new expensive bike from their store!
Hopefully this guide proves usefully to you. A bike is a great investment, they hold their price well at the same time as giving you endless enjoyment. Shop around to find the best deal, and don’t discount a second hand bike - but ALWAYS try before you buy.