There are countless different appliances you can buy to cook with, but ovens in particular are likely to be a once-off purchase. Most people don’t ever replace their ovens, simply because they're built to last. Once you've purchased an oven, it's rare that anything will go wrong and you're unlikely to need another unless you're very seriously redesigning your kitchen.
Pyrolytic ovens are a relatively new type of self-cleaning oven, and they're becoming far more popular these days. Pyrolytic ovens use very high temperatures to oxidise dirt, turning it into ash which you can then simply dust out of the oven. Pyrolitic ovens are becoming much more affordable, and if you're familiar with the effort required to clean filthy oven, you'll appreciate the value of this sort of technology.
Another common technology is catalytic liners. These are linings that are attached to the sides of an oven, which absorb grease and splashes. These splashes can then be burned off at a high heat. Yet another type of self-cleaning oven is the steam oven, which uses a steam cycle to loosen up grime and dirt on the inside of the oven for easy cleaning.
A self-cleaning oven isn't a necessity, but given how long you're likely to have it, you may like to seriously consider the convenience it'll afford you in years to come.
Freestanding/upright or wall ovens
Depending on your tastes, you can either purchase a freestanding oven which sits on the floor and stands at the same height as your benches, or have one built into a wall. Wall mounted ovens are mostly built to standard sizes, and are normally considerably cheaper than similarly specified freestanding ovens. There's a considerably greater choice in terms of special options and sizes when it comes to freestanding ovens and range cookers, so if you're after something less conventional this is likely to be your best option.
Standard oven widths
Conventional ovens sold in Australia, with very few exceptions, are 600mm (60cm) wide from edge to edge on the outside of the oven. Extra wide models are also available, which are 900mm (90cm) across the door. The internal dimensions of the ovens will vary between different models, but obviously if you're serious about entertaining or if you have a very large family, you'll need to consider whether or not an extra-wide oven will better suit your needs - or even whether two ovens or a range cooker might be a better fit.
Range cookers start at about 70cm in width, and get much wider from there. These are very popular in Europe, and often feature several oven compartments.
For conventional ovens, matching cooktops will generally be designed to go with an oven, and will be the same width.
Height of shelves
When you're choosing an oven, look for one that allows plenty of flexibility in terms of shelf configurations, and consider whether or not you're able to cook by placing things on the oven floor. If not, the maximum height of what you can cook will begin on the bottom shelf of the oven.
Gas, electric or dual fuel
The fuel types you choose for your oven will affect how you're able to use it. Gas ovens, for example, don't require the same preheating as electric ovens do - but electric ovens will generally cook more evenly between the top and bottom of the oven.
There are three main choices when it comes to cooktops too - gas cooktops, conventional or ceramic radiant electric cooktops, or induction cooktops (which are also electric). Induction technology cooks by employing magnetic fields that induce heat in steel cookware, so it is the cookware that heats the food rather than direct heat from the cooktop, which remains cool to the touch.
Gas, by contrast to induction and conventional electric hotplates, provides a powerful, immediate and aggressive heat, which you'll almost certainly need if you like cooking with a wok. Induction cookers are generally the most efficient option, as well as the safest (especially if you have small children) although they are normally a bit more expensive.
General oven and cooktop ergonomics
It's very important to think a bit laterally when you're choosing something as big as an oven to make sure you're getting exactly the right product. Be aware of how it'll work in your kitchen. Ask yourself whether or not it'll make the best use of the space available, how it'll impact your benchspace, whether or not the trivets are flat, and if not, how steadily the sorts of pots and pans you use will sit on the cooktop.
Consider where it'll be positioned, how the door of the oven opens, and how that'll affect how you can move around the kitchen. Also find out how easy it is to use before you buy it - especially if you're likely to want to use timers and advanced programming options. You may find knobs or electronic controls unwieldy or even flimsy on the wrong models.
Make sure you also give plenty of consideration to ventilation too - it's no good committing to an oven only to realise you won't be able to properly ventilate it.