you want something that's not croppable by hand. If you can get the bike inside then do, ideally with a ground anchor and some kind of alarm. Be that on the garage door, a disc lock alarm or a trip wire rigged up to a shotgun blank. The first two will wake you up, the latter will get the neighbours to call the police for you. Just ensure the blank is placed where it won't cause powder burns to anyone, including you if you get it off accidentally.
19mm links and a Squire lock with a price tag of around £200 is generally a good sign for a quality chain. I think the Almax series IV is one of these and you need power tools to get through it. Do not buy cheap as you will end up losing your bike to some kids with dirt cheap croppers. There are some Youtube vids testing various chains.
An alarmed disc lock is good but they can be a pain and you get what you pay for. An alarmed one is also useful as if it has a tilt sensor it'll warn you it's on before you try and ride off - you end up on the floor and very embarrassed. You'll find people are actually quite nice when this happens. The reason a disc lock is good is that to just wheel the bike away means practically destroying the front disc and that makes a lot of noise so it needs carrying (or placing on a skateboard) which raises suspicion. An alarmed one is better for obvious reasons. I had a £30 (or maybe £40) Xena one which went off with a sparrow farting and now have an Abus Granit Detecto 8077 (£142) which is excellent - perfect sensitivity and looks / feels very secure and well made. If you're visually serious about your security then a criminal is likely to wonder what other measures you may have in place (tracker, alarm, etc) and so may well pass and move on to another bike. They usually want the easiest steal (which means it comes with the lowest risk) for the highest gain. Kids just want a bike to dick about on so out of sight is out of mind and decent chains will go a long way. Additionally, if you have decent security, don't put the steering lock on. It takes one solid kick to break it, bugger the steering and you have to not only pay for the repairs but for it to be picked up in a van by the garage.
If you get a top box, beware of the weight limit and the weight of the chain you're planning on carrying - often a top box has a limit of 3-5kg and I know mine is way heavier than this. I use a tailpack on the pillion seat for my chain when I have to carry it. If you do this, ensure it's secure as you don't want it mobile and trying to lean you over further in a corner.
Last thing - try insurance quotes with and without security. It may be a similar price without it depending on how much risk you are personally. This means if you forget to secure the bike properly or forget to put the alarm on then you're not going to end up with the insurance company not paying out. I do think that if you say you use a disc lock for example, they expect you to use it every time you leave the bike. So if you stop to get some milk and don't put it on and someone nicks it in the minute you're gone, they could refuse to pay out because you didn't secure it - lying to them and saying you did is all well and good unless there's CCTV, a witness or they recover the bike with the disc lock still in the top box. Plus if you lie to the insurance company then you have to lie to the cops as well and there you can really run into trouble.