L1 visas are usually (well, used to be) quicker to process than H1's, as they don't run through the same system where you have to apply at certain times of the year.
My L1 took 5 months (altho that was nearly 7 years ago now), and that was with the work immigration law firm doing it. I think getting him there by the end of a year might be a push, the company will need to take their lead from whoever's doing their visas. Important to note that once the application is in, you usually can't travel to the US before the visa is issued. There's a ton of expat forums on line that have up to date info on processing and lead times
Your visa will be separate, altho can be submitted at the same time. I can't recommend enough that you get married asap, as the conditions of a spouse visa versus domestic partner are v different. Again, whoever is sorting his visa out should be able to give you the best way to do this.
Most of Manhattan is nice - depends on what sort of area you want to live in, and where your DP;s work is (some of the upper east and west side can be a long commute if working downtown) and how much you want to pay. My friend had a place in flatiron that was a proper 1 bed for about $3200 a month
I will caution that I found a lot of liberties being taken with rental ads and pictures, so do take with a pinch of salt. try and figure out what is important to you - i.e. a proper kitchen (lots of city apartments have microwaves and no ovens for example) or a washing machine (most apartments esp the old ones will not have one in the apartment)
Decluttering before you go is a great idea so you don't ship everything- I'm assuming your DPs office will provide a relocation package that includes shipping? if so, less expense for you. Lots of storage units around the city, prices vary depending on the size you want and where it is. I think we paid about $110 a month for ours (but that was Brooklyn)
i'd recommend you push the firm hard on the relocation package for help with temp accommodation, shipping costs, any one off payments (for large goods / furniture etc)
also get them to confirm whether your DP will be on an expat package or a local one. The former means they will still get UK terms on holidays, contracts etc and usually get flights thrown in, the latter means it will be a local NY 'at will' contract. < This is really important as basically either party can quit the job with a max of 2 weeks notice. If your visa is tied to your job, you have a v small window to have the leave the country