I think the Bachmann 150 will be the plain blue livery.
It will. It's due to be ex London Overground / Sliverlink 150128 in First Great Westerns plain blue livery.
So far, the only models I can sort of help you with for a FGW / Western Region layout are:
FGW Class 57/6 - Limited Edition 57605 in Blue for Kernow Models
FGW Class 57/6 - Limited Edition 57604 in BR 1980s GWR 150 Style Brunswick Green for GWR175
FGW Class 57/6 - Bachmann standard range 57602 in FGW Green / Gold (livery carried until around 2008/10)
FGW HST - Dynamic Lines - Hornby has produced the PCs and several releases of Mk3s since in Dynamic Lines. The power cars are now becoming sought after and expensive. The Mk3s are becoming rather illusive with the TGS and buffet highly sought after and expensive. However, the TS and TF can be found from Hornbys recent batches in Hamleys and some Hornby 'Concession' outfits.
FGW HST - Barbie - Mk3s were produced several years ago, the power cars were the former triang effort. You can get away with mixing Barbie and Dynamic Lines until around late 2008/2009 or 2010 with a Barbie Buffet. You can also get away with a Midland Mainline liveried rake until around 2008 as well with blue power cars.
FGW Night Riviera Mk3 Sleepers - Dynamic Lines - Due for release within this current Hornby year 2014 - 2015 but no Mk3 RFM, TSO or BFO to go with.
FGW Night Riviera Mk3 Sleepers - Green Gold 'Fag Packet' - were produced a good few years ago and now are becoming collectable so watch out. No RFM or TSO go with again. However, Hornby did produce a token Mk3 TGS in 'Fag Packet' to sort of be a mock BFO.
FGW Mk2 TSO / FO & BSO - Again these are becoming rather collectable as they were released around 2005 but can be used for the Night Riviera before the ex Virgin Mk3s became available.
FGW Class 166 - All 166s sought after, Dynamic Lines commanding the most.
FGW Class 143 - To be released by Realtrack Models later this year. Looking like it'll be worth every penny of anyones money having seen the 144s!
Wessex Trains 'Visit Bristol' Class 143 - To be followed up by Realtrack Models at some point after the release of the FGW & ATW Class 143s.
FGW Class 150/1 - To be released by Bachmann later this year.
FGW Class 150/2 - Not yet available in either Local Lines or Plain Blue. Vinyl overlays are available from Electra Railway Graphics. However...
Arriva Trains Wales 150 - Plenty of these went on hire to FGW.
FGW 153 - Sadly also not yet available. I would have expected Hornby to even consider the plain blue trio but disappointingly not. Last produced nearest livery would have been the Wessex Trains Devon and Cornwall black. However again..
London Midland 153 - You should be able to get away with one or two of Hornbys London Midland 153s as per FGWs ex LM twins 325 & 333.
Wessex Trains Devon & Cornwall Black & Gold 153 Has been produced by Hornby, but numbers are now limited and like the expression of 'hens teeth' to find cheaply. Can be used on a FGW region layout as far as 2007/2008. You could also do something I've done and create the 'token' 153361 - A 'token' single car 153 given to the Great Western franchise from the All Wales franchise in return of 12 Class 150/2s, 4? 143s and a few 158s. Arrived into the GW Franchise in Ex First North Western livery.
FGW 158 - Again, not yet available. Could be on Bachmann's radar for future models as they do have a new Class 158 coming out within the next few years.
SWT 158 - Again, not yet available. Could be on Bachmann's radar for future models as they do have a new Class 158 coming out within the next few years. They are producing an East Midlands Trains 158. As far as I know Electra don't do graphics for SWT 158 but im happy to be proved wrong. Bachmann did produce a SWT 159 several years ago but these are now also highly sought after and in the 1st SWT version.
Ex First Transpennine Express 158 - Bachmann has produced the ex Northern Spirit Maroon liveried class 158 in the modified FTPE branding, which ran with FGW from the GW unit lease swapping and start of the franchise in 2006 to refurbishment around mid to late 2008/early 2009, which in turn replaced...
Alphaline Wessex Trains - Silver Class 158s. Bachmann has produced two versions of Bristol based 158s in the original Yellow/Blue & Silver Wales & West Wessex Trains livery - this one with the blue doors then became that of Wales & Borders, whilst Bachmann has also produced the slightly later
Alphaline Wessex Trains liveried 158. This isn't the later final version but does have the more correct Wessex Trains maroon doors. Speaking of Class 158s in the Wessex Trains area...
Central Trains 158 Over the life of the Wessex Franchise, Wessex did receive several class 158s from sister Nat Ex Franchise
Ccentral Trains, mostly having been replaced by Class 170s in the midlands. These would have ran with the franchise until 2006 before being stored, along with Devon & Cornwall Black / Gold 153s 153302, 308 & 374 & Heart of Wessex Pink 153355, in Eastleigh Works.
And finally, another 'Western Region Wanderer' was the
Arriva Trains Wales Ex ScotRail Class 150/2s, which occasionally turned up into Devon when the ATW Penzance / Plymouth to Cardiff and beyond services used to run - ad odd fraction of the Wales & West > Wales & Borders / Wessex Trains split. These are still available from Model Rail / Harburn Hobbies in Edinburgh.
Depending on where your thinking of setting your layout you might want to look at XC models too - Hornby have produced the HST and Bachmann the 220 & 221.
Hope that helps. From another Western Region modeller!
I'll add that to the list above shortly.