The mind boggles at the cost of stocking the fiddle yard 
I can see why it's a long term project.
I'm not even going to think about that aspect! All I'll say is that my tenth Dapol A3 will be with me next month (birthday present) and represents the eighth I've bought either second hand or as a spares/repair for considerably under the RRP (cheapest was a wreck for £10 that I spend another £20, and about a week, in repairs). In case you're interested, the latest one will end up as 4470 'Great Northern' as it was in 1937 with high sided tender but still in original Gresley A1 condition (i.e. no superheater header covers and right hand drive).
The current available and in progress motive power includes:
A1: 4472 'Flying Scotsman' (rebuilt from a wrecked A3 'Papyrus' and paired with a corridor tender back dated to c. 1928-36 condition).
A3: 4480 'Enterprise' (modified 'Papyrus' to right hand drive)
A3: 2500 'Windsor Lad' (Reworked 'Flying Scotsman' with bango dome and paired with a high sided non-corridor tender)
A1: 2562 'Isinglass' (modified 'Grand Parade': purchased spares/repair not yet complete)
A1: 2568 'Sceptre' (as for 'Isinglass' but with westinghouse brake pump dating it to 1933 or earlier: purchased spares/repair not yet complete)
A3: 2573 'Harvester' (as for 'Sceptre' but right hand drive A3: purchased spares/repair not yet complete)
A3: 2744 'Grand Parade' (standard Dapol)
A3: 2750 'Papyrus' (standard Dapol)
A3: 2751 'Humorist' (backdated and repainted BR 'Gladiateur' the only A3 fitted with a double chimney by the LNER)
A4: 2510 'Quick Silver' (standard Dapol)
A4: 4482 'Golden Eagle (standard Dapol)
A4: 4487 'Sea Eagle' (standard Dapol)
A4: 4490 'Empire of India (standard Dapol with incorrect double chimney!)
B17: 2871 'Manchester City' (standard Dapol - not strictly appropriate for Hadley Wood)
C1: 3279 (Gresley rebuild with outside valve gear - 3D printed body on Farish Jubilee chassis with Union Mills tender drive. A very early effort and awaiting a replacement to be built)
C12: 4519 (3D printed body on modified Dapol 14xx chassis and Farish front bogie)
D11: To be 5506 'Butler-Henderson (Union Mills upgrade, awaiting repaint from BR green)
D16/3: not numbered (Union Mills BR version to be repainted)
D49: 234 'Yorkshire' (3D printed body on Dapol Schools chassis with Dapol B1 tender frames and modified M7 bogie moulding - in bits again at the moment!)
J6 (x2): Not numbered (unfinished project 3D printed body on Farish J39 chassi)
J11: Not numbered (Upgraded Union Mills awaiting repaint)
J39: 1856 (Standard Farish)
J50: 617 (3D printed body on custom built chassis from N-Stars in Holland)
K3: 4004 (GNR cab version, 3D printed body on Farish N class chassis with V2 tender frames and modified J39 tender body)
K3: 2425 (LNER built, 3D printed loco and tender body on Farish N class chassis with V2 tender frames)
N2: 4744 (3D printed body on modified Farish 3MT chassis)
N2: 4750 (3D printed body on modified Farish N class chassis)
Q2: Not numbered (Union Mills)
V1: Not numbered (3D printed body on Farish N class chassis - test build in progress before making the print available to buy)
V2: 4771 'Green Arrow' (renumbered Farish - awaiting rebuild/replacement)
V2: 4844 'Coldstreamer' (Standard Farish - awaiting rebuild/replacement)
Total 32 locomotives: Collected or made/modified over a number of years (many when I once had a well paying 9-5 job before I got sick - several years in the past now!) and by constantly searching out the bargains and spares/repairs. That's not including the 3D printed A5 body I have, waiting for a suitable donor chassis to be found! For a general rounding out of the more typical motive power seen through Hadley Wood I really should make a 'standard C1', D2, J3, J52, K2, N1 and O2 as well. It does raise the question of can you really have too many locomotives? I didn't realise just how many I have until I made this list! Rolling stock is another matter and there is no way that I could fill out more than six roads with what I have at the moment!