Okay, so it basically comes down to the paintscheme.
The Grand Plan has holes. So that tends to indicate the direction/faction. I needed to do a Late Republic era League unit, to 'complete the set'. Hence to CamoSpecs! Many League schemes I'd done - Marik Militia, Oriente, Regulans, Orloff. And most Andurien schemes are, agreed, unpaintable by such as me. Yes, I could do Legionnaires, Guard, Knights, etc - but the units didn't appeal. And then I hit the Andurien Rangers - I can do white and green trim, thanks to the liberation of Contrast Paints! Sometimes though, it's just the scheme that looks do-able. No scales, flowers, stars etc - too fussy for me! No shading. But then I saw Neo-Tanuki's excellent-looking Mongrel in Steel Viper Rho colours, and suddenly that's on the to-do list. Steiner Strikers was a mixture of looked distinctive, within my grasp, and filled a hole in the Grand Plan. Seriously, browse the new CSO site - there's tons of new schemes, and many are quite feasible for ordinary folks.
So now the era & faction are sorted out, I look at my minis, and cross-reference with the MUL. I'm in an interesting place - waiting on the KS minis, and wanting to not mix older Alpha Strike plastics & similar sized metals with KS minis. I'm just about out - one lead Panther is about it. But there are some metals which are entirely suitable for use with LS plastics - the Templar, Thanatos, Hollander, Nightstar, Enfield ... rather a Davionista feel? And I have good-ish (if dwindling) stocks of DA plastic vehicles, BA, and infantry. So some paintscheme digging, and in this case word from a friend, and I found the Dawn Guard, and it's perfect for soaking up plastic Sorteks and Partisan AAs and DI Multipurpose VTOLS - even lets me slip some RotS gear in (Simians, Centaurs) just for git & shiggles.
Short form: it helps to have stocks, and the MUL is a joy.
However, stocks and DA plastics sometimes just won't cut it. I've got a Gyrfalcon trinary in metal, all the 'totem' Jade Falcon designs, plus Hels and Grand Summoners. Being non-CONUS, I planned a USD$250 purchase for over a year, finally dumping money to get the Raider fan-funded, and got a lot of metal shipped. Also got minis to bulk up a Hastati Sentinels battalion, vehicles for the MacIntyre House Guards, and some others. Planned, targetted purchasing - having the goals makes this work more efficiently.
The other thing is I don't necessarily want to do the same mini often. (Says the man with 18 Catapults ...) But they were common across several generations of minis. So I tend to get minis I like the look of (no Wyverns, ever! Not a cent for Hussars!), and if I get duplicates it'll often be modded to another config.
And then you discover the mini you really want to use/buy wasn't available in the era you intended ... throw it back, keep it for later, change the era? And sometimes you just go with it - my Late Republic Andurien are rocking a Grand Dragon. Heirloom, inheritance, salvage, or a Pesht Regular who took a wrong turn at Alberquerque? Who cares! :D
So looking in the MUL for that faction/era, knowing what you have, building up a list of wants ... Keeping an eye on ePay for lucky dip (scored 4 resculpted Omnis in one hit recently, 7 Ral Partha leads in another), and saving for the next big purchase (USD$250 is AU$400), tweaking here, polishing there ...
Hope that makes some sense.