'D' makes some valid points. The scariest potential issues are with the later heads. We're talking about late 1972 through 1975 heads with 10 large head attachment bolts and 2 smaller attachment bolts at the front of the head.
These later heads were designed for use with unleaded gasoline, and have a slightly different metallurgy compared to the older heads. The exhaust seats were then flame-hardened to resist valve seat erosion due to the lack of lead as a lubricant. As a direct result, the greater hardness/brittleness of the seats was also responsible for a tendency for the heads to crack. Most prone would be #2 and #3 exhaust seats. If I had to make an educated guess, I'd say that 80% of the later model heads I've removed from core engines were cracked...they really are that bad! They do not tolerate being overheated at all. In general, Opels like to be kept below 195° F, with 210°-215° being the upper limit. As you near 220°-230°, the head's life is seriously at peril! Above that failure is virtually guaranteed.
For me, the best cylinder heads to choose from are the late 1970 through late 1972 heads. To clarify further, these are the heads with four camshaft bearings that are secured with 10 head bolts only. The four cam bearings allow for less camshaft deflection in a performance application, and the exhaust seats of the head were NOT flame hardened therefore the castings are more ductile (less prone to crack). For some reason, 1971 heads appear to be the best to modify for racing use. They are physically easier to port (cast iron material not as hard), and the port walls are usually somewhat thicker too (.220" to .240" usually).
Only slightly less desireable are the 1968-late 1970 heads with three camshaft bearings. They are perfectly adequate for almost any street performance buildup. They also have 10 head bolts, and have non-hardened exhaust seats. They just have three cam bearings. So they are best not used in an all-out racing application.
As a side note, the year of the timing cover being used *should* match the year of the cylinder head. The heights of the covers were different as were the head gasket designs.
For any of the heads without factory flame-hardened seats, I will usually have press-in hardened exhaust seats fitted to allow the use of unleaded fuels without accelerated wear. Try to keep the OD of the seat the same diameter as the valves being used, and do not make the seats deeper than 7/32" (.21875"). This is to preserve the overall integrity of the head casting.
As far as the blocks are concerned, there are some differences over the years. 1975 blocks are a different casting, and have the heater core feed line at the passenger rear side of the block (5/8" hose barb). Normally on the earlier blocks there is a drain plug at this location.
Early blocks (pre 1971) will not have all the bosses cast in for the various oil dipsticks Opel used over the years in various chassis. So you will be relegated to using an aluminum oil pan if you use an early block, since the steel oil pan versions have the dipstick hole cast directly into the block (or at least the bosses are present which can be drilled out). Early blocks also have *slightly* thinner webs at the main cap areas, and smaller main caps as well. I suspect if you were building a 400 hp turbo engine this could be of some concern, but again in normal applications it's nothing to worry yourself with.
If you decide on a 1971-1974 block, then you get the optional dipstick bosses cast into the block itself, the larger main webs, and the larger main caps. Any of the aforementioned blocks can readily accept a .040" overbore, and as long as there is no appreciable core shift....080" to .090" overbores are acceptable too. This would bring the displacement from 1897 cc's (93 mm x 69.8 mm) out to 1979 cc's (95 mm x 69.8 mm which is the factory Opel 2.0 litre displacement) or to 1989 cc's (3.75"/95.25 mm x 69.8 mm) which is a common size used when aftermarket forged pistons are fitted.
HTH,
Bob