To obtain over 260,000 credits easily just take your best car and take it into the Rally d' Capri easy in the Special Conditions hall. Once you obtain the car sell it and you get over 260,000 credits. You can repeat this over and over and soon enough you should be able to afford anything you want.
Enter the European Hall event 1000 Miles! I recommend either the Ginetta G4 '64 (won from Beginner Lightweight K Cup) or the Chevelle SS 454 '70 (won from American Muscle Car Championship). Enter the Cote d'Azur race (I think the last one works just the same). Make sure your car's HP is near or above 200. Put it on B-spec, 5-Push, overtake, 3x speed. Come back after a couple hours (this works best if you leave it overnight). You should have won the race, along with 150,000 credits! This can also be done with the championship race, which is 4 races. 4 races times 150,000 credits plus 300,000 credit prize equals 900,000 credits. The prize car is crappy, but who cares, you have 900,000 credits from doing nothing!
Completed = 169
2.Autumn Ring Days Completed = 281
3.Cathedral Rocks Trail I Days Completed = 239
4.Chamonix Days Complete = 309
5.Circuit de la Sarthe 1 Days Completed = 267
6.Circuit de la Sarthe 2 (Unchicaned) Days Complete = 351
7.Costa di Amalfi Days Completed = 253
8.Cote d Azur Days Completed = 183
9.Deep Forest Raceway Days Completed
= 15
10.Fuji Speedway 2005 F Days Complete = 323
11.Fuji Speedway 2005 GT Days Completed = 141
12.Fuji Speedway 80s Days Completed = 43
13.George V Paris Days Completed = 225
14.Grand Valley Speedway Days Completed = 99
15.Hong Kong Days Completed = 113
16.Ice Arena Days Completed = 155
17.Infineon Raceway Stock Car Course Days Completed = 309
18.Opera Paris Days Completed = 29
19.Special Stage Route 5 Days Completed = 57
20.Suzuka Circuit Days Completed = 71
21.Suzuka Circuit West Course Days Completed = 127
22.Tahiti Maze Days Completed = 197
23.Tsukuba Circuit Wet Days Complete = 337
24.Twin Ring Motegi Road Course Days Completed = 211
25.Twin Ring Motegi Road Course East Short Days Completed = 85