I started collecting my Guard army a couple months ago. I'm actually about at a point where I'm done with it. I might buy a model or two more (Hellhound for example) ebay was my friend in general and allowed me to pick up a pair of Leman Russ for the cost of a single one new, some extra bits let me utilize all the heavy weapons sprues I could and so forth. This let me kit bash the hell out of special characters like the Officer of the Fleet, and I selected some counts as models for certain special characters. I also had Lady Luck and Admiral Awesome smile on me and drop an extra 20 Guardsmen and 6
HWTs in my lap (on the sprue) so I was able to splurge on some things I was going to wait on. Mostly for counts as stuff like using Cadian snipers I picked up for 4$/model as opposed to buying a ratling pack. Or the famous "Vasquez" model instead of Guardsman Marbo, yes she cost a bit more but it's the feel for the army I want that counts right?
I got lucky with vehicles by and large two, winning some lucky auctions for bargain sentinels, Leman Russ, etc. Having an inventive mind and a desire to convert while not really minding being unable to play
GW official tournies also helps. For example for the price of a new Manticore kit I picked up a 1/35 scale M270 MLRS and Flakpanzer Gepard, with a few extra bits they'll make excellent Manticore and Hydra Flak tank stand ins.
So speed of acquisition and assembly depends on time, (I got lucky with some extra free time right at the start), money (I saved up for awhile before I started buying), level of skill (I futzed stuff up a lot >.<

and the occasional bit of Emperor gifted providence and a side order of creativity.
I would emphasize the last, if your
FLGS is good with conversions, and most of the people you're playing with are too I strongly suggest having at it and letting your mind out to wander and explore, you can create some good looking models for a fraction of the cost that still easily fit the fluff provided you're willing to put some time, effort and green stuff in to it.